Could you give us access to your backend so that we could try and see for ourselves what might be the problem? In previous versions of Sitefinity this error happened sometime but starting from 4.3 we thought we eraticated it. As for the Return Code 0 - this eror is reproduceable with other browsers too - it happens when you click somewhere on the backend before the previous service call finished executing.
No problem. If you wish you could give us access to your backend so that we could try and investigate the issue on our side.
What version of Sitefinity are you using? If it is not the latest (4.4) did you try upgrading to it and checking if the error still happens?