Hi Frits Veenema,
To resolve your issue there are two way to do that one open your project folder in terminal or git bash and run these two commands git fetch origin then your branch which you are facing issue use this command git merge origin then your branch name then if you see any conflicts resolve manually then use this command git add . then use git commit -m and your message after -m this occurs when Mendix does not support fast-forward merge and second is method is open your git GUI where you can see what things are not versioned in there also you see if any Conflicts are there are not then pull then changes in git GUI this will resolve your error.
I hope this help.