This is just FYI.
I had a hard time figuring out why my NC instance did not work. I had the green tickmark with no warnings, but files and calendar remained empty. I found that the tideways module was the issue, which I had not enabled in the previous php version. I could not find anything about this so I leave a note here.
You’re the second person in recent weeks impacted by Tideways that I’ve come across. (Unless you’re the same person that submitted a bug report to the NC server repository).
I wonder if Tideways is popular enough to be worth adding to the list of problematic PHP modules in the Admin Manual. 
Out of curiosity does the workaround mentioned here allow Tideways to run with NC? (I’m not even sure if it’s the same underlying cause, but NC certainly uses Symfony).
I have not reported anything, that is someone else.
This issue caused me a lot of headache, there was no reporting in the php and nextcloud logs whatsoever that pointed even remotely to tideways. From my point of view, tideways was highly problematic and deserves a pole position on the list of problematic php modules.
The proposed workaround putting
tideways.features.observer=0
at the end of php.ini or conf.d/20-tideways.ini does not work.