Just saw this issue for the first time today on an instance maintained by my team.
Saw the same issue where first we got the message like
error processing input submit request...
Followed by subsequent attempts to submit the same input prompt giving us
JSON.parse: unexpected end of data at line 1 column 1 of the JSON data
The build does move on an actually do the rest of it's task and end up reporting successful
This warning appears to only show itself to the submitting user if they are using the BlueOcean UI rather than the Classic Jenkins views.
In our testing today, when we instead use one of the classic view UI for input...
...then the submission works fine without any visible errors/warnings and the job runs through to completion.