Unexpected end of input
Indicates a timeout on the check50 server. It has not processed your request. The only advice is "try again later".
error 404
Assuming you mean the "non_existAnt" file test did not pass, this usually indicates some problem with your abs_path from the parse function. There is no "implementation" for error 404, it "comes with" the distro code.
load
This line char c; is problematic. Because of internal casting in fgetc, this can return a "false eof", and almost always does on binary (image) files. Some char in the image files is being cast to the int -1. EOF is defined (in stdio.h) as -1. The loop ends prematurely. If you cast c as an int (instead of char), it should work. This article may help clarify the point.
There may be other problems in your functions. Keep testing in a browser/curl and trying check50 until you get a result. And remember, gdb is a valuable tool for troubleshooting server.