by Emma Richardson.
That is just so strange because basically the error occurs because the "grid" is too big. What you are doing is shrinking the grid and the max_input_vars increases the allowed size of the grid so if one works, both should work. I have mine set to 20000 and have never had an issue through multiple Moodle versions. I just wonder if the setting did not take or if a caching issue was still give trouble. You might check your php info and see if the setting came through.