Edit > Addon Manager > Projects
If you have a project named Gramps with the url
https://raw.githubusercontent.com/gramps-project/addons/master/gramps52
then delete it.
Create a new project named Gramps with the url
https://raw.githubusercontent.com/gramps-project/addons/master/gramps60
This solved the issue for me. Couldn’t find any information that this is a necessary part of the upgrade, though.