can not connect to imap server

[steros]

"Server XXX has disconnected. The server may have gone down or there may be a network problem."

There is no network problem and the server is up and running.
After deleting that account and readding it (without restarting thunderbird) the error is gone.
But after a restart of ubuntu the problem is there again.
Deleting certificates does not help, also setting other security settings does not help.

ProblemType: Bug
DistroRelease: Ubuntu 11.10
Package: thunderbird 11.0.1+build1-0ubuntu0.11.10.1
ProcVersionSignature: Ubuntu 3.0.0-19.32-generic 3.0.27
Uname: Linux 3.0.0-19-generic i686
ApportVersion: 1.23-0ubuntu4
Architecture: i386
BuildID: 20120329060223
Date: Tue Apr 17 10:06:32 2012
InstallationMedia: Ubuntu 11.10 "Oneiric Ocelot" - Release i386 (20111012)
Profiles: Profile0 (Default) - LastVersion=Safe Mode/Safe Mode (Out of date)
SourcePackage: thunderbird
UpgradeStatus: No upgrade log present (probably fresh install)