I have a 1 1/2 year old WordPress Network installation with about 20 sites and over 200 users. Recently I have been unable to add or remove Super Admins to the site because of an Invalid User ID Error.
Whenever I attempt to edit users in the Network Admin panel, the changes fail and I get a page displaying only "Invalid User ID". The only way we can give users access to edit multiple sites on the network is to go the Users Panel on each respective site and manually add that user.
This site is for a student government organization, so user rollover is high and we constantly need to add and remove users and super admins to the site.
Any ideas what could be causing this issue? Is there anywhere I can get more detailed information about the error other than a 3 word message: "Invalid User ID"?