Hi @Martin Janik,
Welcome to Microsoft Q&A!
Which version of Visual Studio are you using?
Please ensure you are connected to TFS in VS.
Then, open the VS and go to Tools > Option > Source Control > Plug-in Selection > Current source control plug-in: Visual Studio Team Foundation Server.
Besides, please check if there is a .git folder in the source directory. If it exists (it may be hidden), please backup and delete it.
Sincerely,
Anna
If the answer is the right solution, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment". Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.