EDIT: I have observed in all my local sites that the USER admin form is unable to save changes, ie add or remove a field, and when I change to the site form I receive a message: "The most recent request was denied because it contained an invalid security token. Please refresh the page and try again.
EDIT OF MY EDIT: I did what Lionel said re max_input_vars and all is good. Thought it was a new problem as never experienced it before and my setup is the same.
Hi Oscar
1
So do you get the same 'save' issue with a fresh installation of Joomla. If not, compare the difference ie other extensions loaded, changes made etc. Sounds pretty odd and broken.I had a drama a long time ago that I thought was between a slideshow module by someone and a component by someone else. It turned out the new Joomla version didn't have the right javascript version (or something like that...)
2
Where to store the fields value:
In the seblod user form, the storage for usergroups is...
Standard | User | groups
So that is the answer, too.
For your select field or whatever you have used set storage to the same
Bucklash