Maybe you ran out of space in the inbox? Are you using the inbox to store messages? It was not meant to be used that way. See this page: http://kb.mozillazine.org/Keep_it_working_%28Thunderbird%29

I have this problem too, but my Inbox is empty. There are 125 emails waiting on the server, including one i just sent from another address. The site you recommended is "having difficulties".

May 4 I have now accessed the KB article above. None of the recommended actions made a difference. I deleted about half the 130 emails awaiting download from web-mail, but still got the same error. I can't clean out the inbox because I can't download emails yo it :-( I have 3 other addresses operating under the same server satisfactorily.