Hello Christian and Barto,
I've just tried to reproduce your error and everything works fine for me, a vm with the version 3.1.4 and then upgraded to 3.1.5.
Could be that you have the administration web cached in the browser? Can you try to clean the browser cache and try again please?
If not, can you give us more info about the upgrade path? Did appear any error in the zentyal.log file after it?
Thanks for your time
Best