Have you got any common folders? (folders not belonging to any account) If not, then close TB and delete the account.flb file in your mail directory (be careful, not the one in your account directory, your account directories are subdirectories of your mail directory)
Open TB and see whether that helped.
When you're running TB pro with an encrypted message base, the file is called account.efb

Just in case of problems you might rename the file in stead of deleting it.

Just to clarify, the "mailfolder" directory on the server has subfolders by the account name. And every account folder has subfolders by the name of attach, inbox, outbox, sent, etc.

The funny thing is that I manage three accounts and the problem appears only with one account! The other two accounts display normal ... Can I post a screenshot here? If yes how?

Kind regards
Frank

Now I've seen you're folder structure I'd suggest the following:

Create a new account with the same settings as your corrupt account (address, server, etc)
Copy all messages from your old account to your new account and afterwards delete your old accounts.
Don't forget to copy any existing filters, quick templates, etc.

The reason that I didn't suggest this right away was that I thought you might have an account like mine, with some 150 folders, that makes it a bit messier to copy everything.