R-click on the folder BirdLife-ÖgOF > Properties > General Info > Location >> this will give you the information whether this is a IMAP or POP folder and where in the profile it is located (and the path to it if POP). Open the Thunderbird profile folder https://mzl.la/2cYKHaB and navigate to a BirdLife-ÖgOF.sbd folder. Verify if there is also a BirdLife-ÖgOF file without any extension in the same location. Open the BirdLife-ÖgOF.sbd folder and show us a screen shot of its contents.

I cannot give more precise directives to find the BirdLife-ÖgOF.sbd folder in the profile folder since you did not give precise information.

I'm sorry but your reply is too complicated for me to understand. Can I just uninstall and then re-install Thunderbird and hope to be able to recover at least some of the data?

Uninstall and reinstall the application Thunderbird won't change anything. Your email data are stored in the profile folder of Thunderbird and not in the application.

By chance I noticed that I exported data to a zip-file called Thunderbird_profile_backup in August last year. It's 5.6 GB in size. Can I use it to restore data?

Can I use it to restore data?

You certainly can use it but it won't contain the newer messages since last August. Unzip the file and show us a screen shot of its contents. It will be much easier to tell you how and where to integrate folders in your actual profile folder.

Unfortunately the zip file seems to be corrupt. Error message when I try to open

I'm sorry but your reply is too complicated for me to understand.

I cannot see your screen, otherwise your problem certainly would have already been solved. Are you able to create screen shots? I sent you a message and a SMS. Did you read them?