What is your anti-virus software?

Microsoft security essentials, no further AV software installed. One of the computers involved is a very fresh machine. I do not run TB addons.

Try to start *Windows* in safe mode with networking enabled.

- Win8 http://windows.microsoft.com/en-US/windows-8/windows-startup-settings-including-safe-mode

- Win7 http://windows.microsoft.com/en-US/windows7/Start-your-computer-in-safe-mode

- XP http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/boot_failsafe.mspx

Still in Windows safe mode, start Thunderbird in safe mode.

- https://support.mozilla.org/en-US/kb/safe-mode

Does the problem go away?

The problem persists, also with Win7 and Thunderbird in safe mode. Same when I disable the network connection (by hard switch on the notebook).

Are all computers connecting to the same account?

Yes they are.

If only one computer is connected does it crash? A single IMAP maiol account is not meant to service a whole sales department, but that appears to be what people are trying rather unsuccessfully to do.

Usually, only one computer is connected at a time, and this was also the case when the problem first occurred. Occasionally the two computers have been online at the same time, but it has not caused any trouble then. However, a GMX mail app on my Android phone is constantly connected. Could it be interfering? This config has been running for many months, and on earlier devices even years. I suspect some form of IMAP induced corruption in my mail profile folder, is there a way to troubleshoot it? Thanks

Compact your folders. File menu (alt+F and select compact.

Compact will remove corruption as it reads the underlying files and makes a new file. (assuming you download and synchronize messages on the computer.)

Turning off the message app on the phone might allow a little experimentation.

You have basically ruled out anything in the way of corruption by creating a new profile. So I think there must be an external source.

Disable MSE and see if disabling it helps. Run the windows disk cleanup tool. In case we are running into weird temp file issues. Scan the disk for physical errors. Nothing is impossible. Go to getthunderbird.com and download and install a fresh copy. Again just to be sure.

Compacting folders seems like a good idea, however, since thunderbird freezes after a few seconds, is there another (external) way to do this? Thanks

at a command prompt cd Thunderbird installation directory ./thunderbird -offline

And start in offline mode. We will see if it is something in the networking components that we are to look at.

Hi all, thanks for your responses. It turned out (after installing a complete new thunderbird on another machine and let it sync with imap) that in the automatic-Spam folder of GMX there was one single e-mail that caused Thunderbird to freeze while loading this mail upon boot. I cleaned up the folder (containing hundreds of mails) via webmail and deleted the Spam folder manually from my profile folders, this solved the problem om all computers. Unfortunately, I could not find out which e-mail or attachment in perticular caused the problem, but it might have been infected by something which was not detected by either the GMX spamfilter or MS Security Essentials.