Critical error

I am getting errors on several websites after adding/removing categories using Quick Edit on an existing entry. The error is different depending on the hosting … on one, ‘There has been a critical error on this website’ appears in the Download Title column and the other just hangs with the spinner spinning . In both cases, the error disappears when the page is refreshes & it looks as if the categories are updated OK, but the error is quite disconcerting.