Perhaps you no longer have a "keychain" that previously entered the account and password for you?
I have been wondering that too, but nothing has changed in the keychain that I initiated. Everything else that uses it works fine. I did discover that moving the files to another user will enable them to be opened. I have removed all the preference files and that doesn't help (assuming they all have filemaker in the name). At least I have recovered the information from the tables.