great to hear 
Would like to diagnose what went wrong
that’s the hard part… the snap magic happens when the nextcloud fixer does the database stuff for the user. which is likely what happened when you rebooted and thus restarted the snap. restarting the snap would have sufficed.
the snap was designed with IoT in mind… very little to no user interaction required. its a default stable upstream Nextcloud packed into a neat easy to install package with no bells and whistles. as soon as you start fiddling with executable binaries, you’re defeating the object of snap confinement which is meant to be a secure, sandboxed, containerized application isolated from the underlying system and from other applications.
this is why we’ve tried to enable users to host services like Talk:HPB, CODE, AI, imaginary, whiteboard etc. as separate services for the snap.
which will be the case if you stick to the default stable install 
and host services requiring executable binaries separately.
i’ve tried to reconstruct your issue on my testing environment with a fresh 31.06 incl. assistant upgrading to 31.07 and upgrade worked fine - so reconstruction failed… we’d need more details.
please mark this topic resolved if your issue is resolved. thanks