I use t-bird with my work imap server all the time. No problems here. It's certainly a configuration problem, either with the client (t-bird) or with the imap server. I would start at the imap servers log files, and turn on verbose mode there if you can.

On the client side (t-bird), you could try setting up a new email account (in t-bird only, using your existing imap server account name) and see if it works. If it does, then there's a setting difference between the two accounts. If not, I'm not sure a new client will help you. It seems like you need to sort this out first. (turn off the 'check for new mail every x minutes' in the primary account, when testing the secondary account name in t-bird. You don't want to end up with two competing imap clients.)

Alternatively, you could use t-birds profile manager to set up a totally separate profile and do it there. Though it might be harder to compare settings that way, which is why I suggested the other method first. Just acreate a new email account in t-bird, including a new imap server in your existing profile, so you don't end up re-using the current/broken one. I have two t-bird client "accounts" set up for my work imap email depending on how I connect. The normal method, and the secondary and rarely used method, which uses a different set of proxies to get in. It works fine. When set up properly, it doesn't look any different to anybody I send mail to since you control what goes in the "From" line. They just have different t-bird account names and different server settings.

-David