I had that yesterday too, when I made a post. It happened when I was trying to load a photo. That was in Safari so I went to Chrome it did it a couple of times then was OK. So far today no problem, either in Safari or Chrome.
Oops! Spoke too soon. Just been delving into some old posts and got this (see screen shot below)when I clicked on a post. However next time I clicked on the post it all was OK.
CrappieHappy is correct. If it is intermittent: where it gets the error once, then no error next try, then an error and so on..... it is usually a server side issue. Not much i can do but report the problem and wait for their tech guys to make the fix or finish what ever updates they are working on.
I haven't been able to log in until just now and have not gotten the message yet, but that doesn't mean much. Just have to wait it out. I'll move this down to the Error Reports.