Weird question, but have you tried logging out of Studio Pro and logging in again? That has usually solved the problem for me in the past.
Do you happen to have a .lock file in your_mendix_project/.git/ref/heads/newly_created_branch_name.lock?
(or any .lock file lost in the .git hidden folder)
if yes, then delete it and try again.
(probably you need to do this with Studio Pro turned off)