Cannot add Related Items

After more than 10 years of using Omeka successfully (currently Version 2.7.1) , of late I am unable to add any Related Items. Colleagues on their machines trying to add items to our site get the same message. I enter the item number, select Is Related To, and when I try to save, I get this not-very-useful error message: " Omeka has encountered an error." Any suggestions?

After reading “Error Messages in Relation to Items and Plugin Issues,” I tried following that procedure. I have been unable to change permissions so that I get a detailed error message.

You’ll have to do the retrieving error messages steps you refer to in order to get the necessary information here.

If the “Display Error Messages” steps don’t work, you can do the “Activate Error Logging” steps instead to log to a file.