Based on the picture the error follows from the description.
If you have access to the web server logs (httpd/apache, ngnix) and PHP logs, if logged separately, those would help you pin point the problem.
An error code of 500 means there is an internal server error. So code on the server is not liking something.
Have you validated that the post size and max upload size are capable of handling the data you are sending it?
Have you validated that the processing time is long enough to handle the different clients?