well, that means the webserver (bartender) is sick -- there is nothing you can do about it (problem at their end)

The problem might be as simple as no proper error checking, so when you send the webserver(bartender) questionable json values, its just explodes into 500 instead of doing what it was supposed to do

There is just no way of guessing ... other than recording/reproducing verbatim existing browser sessions, in most every conceivable detail ... all those various headers you're not including like X-JNA....