AdHoc command AuthenticateUser should work if the UserProvider is readonly.
The AdHoc command AuthenticateUser doesn't work if the UserProvider is readonly. As the command doesn't actually change anything, this restriction can be dropped.
There's a comment in the code that reads:
Check if groups cannot be modified (backend is read-only)
I suspect that this is caused by copy/pasting from another class.
Removed the block that does the check.