When you are already using the code itself, and something breaks between versions, you can directly ask the developers on the bugtracker Issues · nextcloud/desktop · GitHub
Thanks - I tried that but the issue was closed as “not a bug”.
But I have gotten 3.14.1 working. I went through my entire system and deleted all files with nextcloud in the name. Then I rebuilt 3.14.1 again, and to my amazement it started working. My theory is that there was an older nextcloud library file that was taking precedence.
But that is just a guess - it is not worth it to try and recreate the problem to do further debugging.