I'm having some trouble connecting to a MySQL 5.1 server on an RDS instance on AWS from my laptop.

The detailed description of the problem is here: https://forums.aws.amazon.com/thread.jspa?messageID=323397

In short: I have 2 MySQL servers, both with the same db configuration and firewall (security group) configuration. One of them works fine: I can connect to it from my EC2 instances (ie, from inside the AWS cloud) and from my laptop. The other one doesn't: I can connect from my EC2 instances but not from my laptop.

The symptom: a connection attempt from my laptop just hangs, and then times out, as if there was a firewall blocking me (ie, silently dropping my SYN packets).

I must say that everything has been working fine for a very long time, and this problem began suddenly, 3 days ago, without any modifications to DB parameters or the security groups.

My current analysis of the situation:

• The firewall (ie, security group) cannot be the problem: both MySQL servers share the same firewall configuration -- I can connect to one of them but not to the other. Later on, I even added a rule to allow inbound connections from 0.0.0.0/0 (ie, I turned off the firewall), and nothing. Oh, I also created a new, fresh security group and changed this instance's SG to the new one (to which I first added my ip address, and then 0.0.0.0/0) but still nothing.
• The credentials cannot be the problem: I use the same from my laptop and from my EC2 instances -- and the user (which is what Amazon calls master user), in the database, has a host of '%'.
• MySQL is not blocking my IP due to, say, too many failed connection attemps: I've FLUSH HOSTS on the database, and also I tried to connect using many different source IP addresses, even from all around the world through a VPN proxy service.

What could I be missing?

I'm asking here because it's been about 36 hours since I've posted on AWS forums but got no answer at all over there... someone here might have a solution!

Any input is really appreciated, I'm out of ideas. Thanks!