Have you done a hard disk check recently? That sounds like a rather drastic loss of data has happened. Both the program and its profile…

Zenos said

Have you done a hard disk check recently? That sounds like a rather drastic loss of data has happened. Both the program and its profile…

I was able to retrieve the local folders, they were still on the harddisk, but the newly installed Thunderbird was using new (not the existing) paths. Manually I was able to redirect the new Thunderbird to the old paths. Nevertheless, I was not able to identify why Thunderbird was re-installed and created new paths, overlooking the old local folders. I also was not able to repair the paths in a way that everything (all e-mailaccounts, all agendas, tasks, local folders) was restored in full. So I have the local folders, it is not a harddisk problem, the problem is that for whatever reason Thunderbird re-installed itself and started from scratch.

No the profiles.ini file that points to the profile was damaged, probably by a system crash when it was being updated.

anything that you have done that did not correct the location of the actual profile is likely to come back and bite you when you try and move to a new computer.

This article cover how to use the profile manager to create a new profile, using the old location and restore things. I hope you did something very similar. http://kb.mozillazine.org/Moving_your_profile_folder_-_Thunderbird