Unfortunately I can't tell much from the screenshot and I wasn't able to track down the corresponding error in our logs. Is there any additional information you can provide? Are you able to access any error information for the xhr request via Firebug or Chrome Dev Tools?
Hi! We are experiencing the same issue since a couple of weeks now. Sometimes it worked after retrying two or three times. The last time it occurred (just minutes ago) this is the error code that the ajax response returned (and it seems it doesn't allows us two merge if we retry like other times):