Ajax error Result Code: 200 StatusText: OK ResponseText: empty

when i use the importer to import a csv with many lines 1300+ i get this error
Ajax error Result Code: 200 ... StatusText: OK ResponseText:
the problem is that the error repeats in a strange way..
if i take a csv of 300 lines and add it...the error will mostly show...but in a few case it dosent, without changeing the csv or anything...
if i import again many time the same csv, sometimes will work....

another strange thing about this error is that it somehow brakes the importer, if i try to use the importer after he gives the error
the button "import" gets disabled and the text on it changes to something like "importing33%"

i've tryd to solve this bug myself but i cant find a pattern or when or what makes this error popup, usualy this error gives a repsonsetext thats shows wheres the problem...but i dont get any text