Everything was working just great, now all of a sudden I can't connect to the server anymore.
The usual cause of this is that you have blacklisted your own IP address. By default FileCOPA is configured to blacklist IP addresses if 3 incorrect login attempts are made within a 1 minute period. If you have done this, then your own IP address will have been added to the blocked IP addresses list.
Go to the Security Page of FileCOPA and check to see if your IP address is in the list. If it is, remove it, click apply, then try connecting again.
