As root, using a reasonably default .profile and .shrc and an empty tmux.conf, I am unable to split the window horizontally. There are a number of cases to consider so I'll list them clearly.

• Using the keybinding + empty configuration: nothing happens
• Using the keybinding + my configuration: a bell is generated, nothing else; occasionally, the split will appear and disappear immediately (maybe it always does this, but I'm connecting over ssh so it may not make it through)
• Using tmux split-window -h with any config: tmux immediately exits

I've posted here in order the server and client verbose logs generated by tmux -v during the third case:

server started, pid 9523
socket path /tmp/tmux-0/default
new client 7
got 100 from client 7
got 101 from client 7
got 102 from client 7
got 103 from client 7
got 104 from client 7
got 105 from client 7
got 105 from client 7
got 105 from client 7
got 105 from client 7
got 105 from client 7
got 105 from client 7
got 105 from client 7
got 105 from client 7
got 105 from client 7
got 105 from client 7
got 105 from client 7
got 105 from client 7
got 105 from client 7
got 105 from client 7
got 105 from client 7
got 105 from client 7
got 106 from client 7
got 200 from client 7
cmdq 0x801c6e080: new-session (client 7)
new term: xterm
xterm override: XT 
xterm override: Ms ]52;%p1%s;%p2%s
xterm override: Cs ]12;%p1%s
xterm override: Cr ]112
xterm override: Ss [%p1%d q
xterm override: Se [2 q
new key Oo: 0x1021 (KP/)
new key Oj: 0x1022 (KP*)
new key Om: 0x1023 (KP-)
new key Ow: 0x1024 (KP7)
new key Ox: 0x1025 (KP8)
new key Oy: 0x1026 (KP9)
new key Ok: 0x1027 (KP+)
new key Ot: 0x1028 (KP4)
new key Ou: 0x1029 (KP5)
new key Ov: 0x102a (KP6)
new key Oq: 0x102b (KP1)
new key Or: 0x102c (KP2)
new key Os: 0x102d (KP3)
new key OM: 0x102e (KPEnter)
new key Op: 0x102f (KP0)
new key On: 0x1030 (KP.)
new key OA: 0x101d (Up)
new key OB: 0x101e (Down)
new key OC: 0x1020 (Right)
new key OD: 0x101f (Left)
new key [A: 0x101d (Up)
new key [B: 0x101e (Down)
new key [C: 0x1020 (Right)
new key [D: 0x101f (Left)
new key OH: 0x1018 (Home)
new key OF: 0x1019 (End)
new key [H: 0x1018 (Home)
new key [F: 0x1019 (End)
new key Oa: 0x501d (C-Up)
new key Ob: 0x501e (C-Down)
new key Oc: 0x5020 (C-Right)
new key Od: 0x501f (C-Left)
new key [a: 0x901d (S-Up)
new key [b: 0x901e (S-Down)
new key [c: 0x9020 (S-Right)
new key [d: 0x901f (S-Left)
new key [11^: 0x5002 (C-F1)
new key [12^: 0x5003 (C-F2)
new key [13^: 0x5004 (C-F3)
new key [14^: 0x5005 (C-F4)
new key [15^: 0x5006 (C-F5)
new key [17^: 0x5007 (C-F6)
new key [18^: 0x5008 (C-F7)
new key [19^: 0x5009 (C-F8)
new key [20^: 0x500a (C-F9)
new key [21^: 0x500b (C-F10)
new key [23^: 0x500c (C-F11)
new key [24^: 0x500d (C-F12)
new key [25^: 0x500e (C-F13)
new key [26^: 0x500f (C-F14)
new key [28^: 0x5010 (C-F15)
new key [29^: 0x5011 (C-F16)
new key [31^: 0x5012 (C-F17)
new key [32^: 0x5013 (C-F18)
new key [33^: 0x5014 (C-F19)
new key [34^: 0x5015 (C-F20)
new key [2^: 0x5016 (C-IC)
new key [3^: 0x5017 (C-DC)
new key [7^: 0x5018 (C-Home)
new key [8^: 0x5019 (C-End)
new key [6^: 0x501a (C-NPage)
new key [5^: 0x501b (C-PPage)
new key [11$: 0x9002 (S-F1)
new key [12$: 0x9003 (S-F2)
new key [13$: 0x9004 (S-F3)
new key [14$: 0x9005 (S-F4)
new key [15$: 0x9006 (S-F5)
new key [17$: 0x9007 (S-F6)
new key [18$: 0x9008 (S-F7)
new key [19$: 0x9009 (S-F8)
new key [20$: 0x900a (S-F9)
new key [21$: 0x900b (S-F10)
new key [23$: 0x900c (S-F11)
new key [24$: 0x900d (S-F12)
new key [25$: 0x900e (S-F13)
new key [26$: 0x900f (S-F14)
new key [28$: 0x9010 (S-F15)
new key [29$: 0x9011 (S-F16)
new key [31$: 0x9012 (S-F17)
new key [32$: 0x9013 (S-F18)
new key [33$: 0x9014 (S-F19)
new key [34$: 0x9015 (S-F20)
new key [2$: 0x9016 (S-IC)
new key [3$: 0x9017 (S-DC)
new key [7$: 0x9018 (S-Home)
new key [8$: 0x9019 (S-End)
new key [6$: 0x901a (S-NPage)
new key [5$: 0x901b (S-PPage)
new key [11@: 0xd002 (C-S-F1)
new key [12@: 0xd003 (C-S-F2)
new key [13@: 0xd004 (C-S-F3)
new key [14@: 0xd005 (C-S-F4)
new key [15@: 0xd006 (C-S-F5)
new key [17@: 0xd007 (C-S-F6)
new key [18@: 0xd008 (C-S-F7)
new key [19@: 0xd009 (C-S-F8)
new key [20@: 0xd00a (C-S-F9)
new key [21@: 0xd00b (C-S-F10)
new key [23@: 0xd00c (C-S-F11)
new key [24@: 0xd00d (C-S-F12)
new key [25@: 0xd00e (C-S-F13)
new key [26@: 0xd00f (C-S-F14)
new key [28@: 0xd010 (C-S-F15)
new key [29@: 0xd011 (C-S-F16)
new key [31@: 0xd012 (C-S-F17)
new key [32@: 0xd013 (C-S-F18)
new key [33@: 0xd014 (C-S-F19)
new key [34@: 0xd015 (C-S-F20)
new key [2@: 0xd016 (C-S-IC)
new key [3@: 0xd017 (C-S-DC)
new key [7@: 0xd018 (C-S-Home)
new key [8@: 0xd019 (C-S-End)
new key [6@: 0xd01a (C-S-NPage)
new key [5@: 0xd01b (C-S-PPage)
new key [I: 0x1031 ((null))
new key [O: 0x1032 ((null))
new key OP: 0x1002 (F1)
new key OQ: 0x1003 (F2)
new key OR: 0x1004 (F3)
new key OS: 0x1005 (F4)
new key [15~: 0x1006 (F5)
new key [17~: 0x1007 (F6)
new key [18~: 0x1008 (F7)
new key [19~: 0x1009 (F8)
new key [20~: 0x100a (F9)
new key [21~: 0x100b (F10)
new key [23~: 0x100c (F11)
new key [24~: 0x100d (F12)
new key [2~: 0x1016 (IC)
new key [3~: 0x1017 (DC)
replacing key OH: 0x1018 (Home)
replacing key OF: 0x1019 (End)
new key [6~: 0x101a (NPage)
new key [5~: 0x101b (PPage)
new key [Z: 0x101c (BTab)
replacing key OA: 0x101d (Up)
replacing key OB: 0x101e (Down)
replacing key OD: 0x101f (Left)
replacing key OC: 0x1020 (Right)
spawn: /bin/sh -- 
session 0 created
writing 207 to client 7
got 208 from client 7
input_parse: '#' ground
input_parse: ' ' ground
keys are 7 ([?1;2c)
received service class 1
complete key [?1;2c 0xfff
keys are 1 (t)
complete key t 0x74
input_parse: 't' ground
keys are 1 (m)
complete key m 0x6d
input_parse: 'm' ground
keys are 1 (u)
complete key u 0x75
input_parse: 'u' ground
keys are 1 (x)
complete key x 0x78
input_parse: 'x' ground
keys are 1 ( )
complete key   0x20
input_parse: ' ' ground
keys are 1 (s)
complete key s 0x73
input_parse: 's' ground
keys are 1 (p)
complete key p 0x70
input_parse: 'p' ground
keys are 1 (l)
complete key l 0x6c
input_parse: 'l' ground
keys are 1 (i)
complete key i 0x69
input_parse: 'i' ground
keys are 1 (t)
complete key t 0x74
input_parse: 't' ground
keys are 1 (-)
complete key - 0x2d
input_parse: '-' ground
keys are 1 (d)
complete key d 0x64
input_parse: 'd' ground
keys are 1 ()
complete key  0x7f
input_parse: '' ground
input_c0_dispatch: '
input_parse: '' ground
input_parse: '[' esc_enter
input_parse: 'K' csi_enter
input_csi_dispatch: 'K' "" ""
keys are 1 (w)
complete key w 0x77
input_parse: 'w' ground
keys are 1 (i)
complete key i 0x69
input_parse: 'i' ground
keys are 1 (n)
complete key n 0x6e
input_parse: 'n' ground
keys are 1 (d)
complete key d 0x64
input_parse: 'd' ground
keys are 1 (o)
complete key o 0x6f
input_parse: 'o' ground
keys are 1 (w)
complete key w 0x77
input_parse: 'w' ground
keys are 1 ( )
complete key   0x20
input_parse: ' ' ground
keys are 1 (-)
complete key - 0x2d
input_parse: '-' ground
keys are 1 (h)
complete key h 0x68
input_parse: 'h' ground
keys are 1 (
)
complete key 
 0xd
input_parse: '
' ground
input_c0_dispatch: '
input_parse: '
' ground
input_c0_dispatch: '

new client 13
got 100 from client 13
got 101 from client 13
got 102 from client 13
got 103 from client 13
got 104 from client 13
got 105 from client 13
got 105 from client 13
got 105 from client 13
got 105 from client 13
got 105 from client 13
got 105 from client 13
got 105 from client 13
got 105 from client 13
got 105 from client 13
got 105 from client 13
got 105 from client 13
got 105 from client 13
got 105 from client 13
got 105 from client 13
got 105 from client 13
got 105 from client 13
got 105 from client 13
got 105 from client 13
got 106 from client 13
got 200 from client 13
cmdq 0x801c6e160: split-window -h (client 13)
spawn: /bin/sh -- 
writing 203 to client 13
input_parse: '#' ground
input_parse: ' ' ground
input_parse: '#' ground
input_parse: ' ' ground
lost client 13
session 0 destroyed
writing 203 to client 7
got 205 from client 7
writing 204 to client 7
lost client 7

got 207 from server
got 203 from server
got 204 from server

There are some other peculiarities:

• With a newly created user (from which I overwrote root's .profile and .shrc, tmux works perfectly.
• Occasionally (twice out of the 50 or so times I've tested it), the splitting will work fine once in a session. (This happened for example when I ran ktrace on tmux, which I can also post)
• To explain the 'suddenly' part of the title: when I started my newly updated mysql56-server, tmux immediately exited and lost the session.

Recently I changed architectures, from FreeBSD 10.0 i386 to amd64, and I am still working through shared library incompatibilities. I suspect that this could be involved, but I can't imagine how an incompatibility of this sort could result in such a specific, isolated failure.