Model: QNAP TS-410 Turbo NAS
Firmware version: 3.2.5 Build 0409T
Issue: Each day, users connect to share folders on the NAS system and have read/write permissions for the share folders to which they need access. However, it often asks them for their log-in details and - when provided with right (or wrong) credentials for a user with read/write permissions - it denies them access.

I've checked the logs and I keep seeing the following warnings:

2011-11-23    16:26:29    System    127.0.0.1   localhost    Re-launch process    [rpc.mountd].  
2011-11-23    16:26:16    System    127.0.0.1   localhost    Re-launch process    [proftpd].  
2011-11-23    16:25:30    System    127.0.0.1   localhost    Re-launch process    [rpc.mountd].  
2011-11-23    16:25:15    System    127.0.0.1   localhost    Re-launch process    [proftpd].  
2011-11-23    16:24:33    System    127.0.0.1   localhost    Re-launch process    [rpc.mountd].  
2011-11-23    16:24:21    System    127.0.0.1   localhost    Re-launch process    [proftpd].  
2011-11-23    16:23:37    System    127.0.0.1   localhost    Re-launch process    [rpc.mountd].  
2011-11-23    16:23:25    System    127.0.0.1   localhost    Re-launch process    [proftpd].  

They seem to occur per minute but I am uncertain about whether or not they are relevant to this issue. The "Login failed" warning has also displayed in the system connection logs which tells me when and which user was unable to log in, as shown below:

2011-11-22    16:11:07    Administrator    192.168.0.xx    computer-01    SAMBA    ---              Login Fail  
2011-11-22    16:11:07    Administrator    192.168.0.xx    computer-01    SAMBA    ---              Login Fail  
2011-11-22    16:11:06    Administrator    192.168.0.xx    computer-01    SAMBA    ---              Login Fail  
2011-11-22    13:46:14    administrator    192.168.0.yy    ---            HTTP    Administration    Login Fail  
2011-11-22    13:46:09    administrator    192.168.0.yy    ---            HTTP    Administration    Login Fail  
2011-11-21    15:17:22    user             192.168.0.zz    computer-02    SAMBA    ---              Login Fail  
2011-11-21    15:17:18    user             192.168.0.zz    computer-02    SAMBA    ---              Login Fail  
2011-11-21    15:17:17    user             192.168.0.zz    computer-02    SAMBA    ---              Login Fail  

I've researched this on Google and the QNAP forums and have not come up with a resolution as yet.