Not really enough to go on in that log. Your next clue will be in the IIS logs to see what it did with the post request. You can also look inside the config.ini file for the directory path to the reports_custom and reports directories and then verify that they exist in the proper locations. Has this ever worked? If so then what has changed since this last time this was working?
I suggest you open up a case so we can take a look at this.