@danielh The errors we've seen happen on exporting encrypted files. ODK Briefcase Not Setting Up Properly after Download and https://github.com/opendatakit/briefcase/issues/627 are the places we've documented it.
I put that warning in the docs because we haven't had a chance to test OpenJDK rigorously. Naturally we want to support OpenJDK, but if we can't get it to work, Oracle Java 9 is an easy upgrade and is free and will continue to be free.
We do need help, so maybe you can try OpenJDK and see if it works?