"grave and unexpected" 500 error on POST using Multipart form-data

[astrodb]

I have multiple Vollt based TAP services running in docker containers on the same host, each visible on a different port, all made to look "clean" through an Apache proxy. A user recently notified us that POST requests to TAP sync/async in multipart/form-data content are failing with a "INTERNAL SERVER ERROR! Sorry, this error is grave and unexpected. No explanation can be provided for the moment". He noted that this type of request works on other Vollt based TAP services, so it is likely a problem with ours. I've checked the tomcat logs in the container, but don't see any obvious messages of what has gone wrong. I've also verified the error occurs through the proxied URL and through directly calling the aync/async endpoints on the server port.

In short, this works: curl --url http://tap.roe.ac.uk/wsa/sync --data REQUEST=doQuery --data LANG=ADQL --data QUERY='select top 3 table_name from tap_schema.tables'

But this gives an error: curl --url http://tap.roe.ac.uk/wsa/sync --form REQUEST=doQuery --form LANG=ADQL --form QUERY='select top 3 table_name from tap_schema.tables'

Is this likely configuration error on my part? If so, can you please point me to where I can fix it?