A colleague forwarded me the following error message:
The problem occured when he tried to create a branch, but only on this specific project we had issues.
Time to log on the TFS server and check the Event Log. There I found the following error message:
DESCRIPTION: SQL Server Assertion: File: , line=951 Failed Assertion = 'IS_OFF (BUF_MINLOGGED, m_buf->bstat) || pageModifyType != PageModifyType_Contents || GetPagePtr ()->IsTextPage ()'. This error may be timing-related. If the error persists after rerunning the statement, use DBCC CHECKDB to check the database for structural integrity, or restart the server to ensure in-memory data structures are not corrupted.
This error message brought me to the following Connect ticket: https://connect.microsoft.com/SQLServer/feedback/details/3005568/sql-server-2016-merge-statement-fails-when-running-db-in-simple-recovery-model
I first tried to install all possible updates but no luck, the issue remained. Then I saw that in the ticket they mention switching to the Full recovery model. I tried that and it seemed to work.