On a recent deployment I found this issue on a Nextcloud 17.0.1 install using mysql-57, nginx, php72. Pretty much everything works, but within the text editor app, the app loads but the text/menu does not. I checked out the dev tools in my browser sends back a 500/“Could not create path” response. In comparing this to a working installation, I cant seem to find any sort of version/configuration difference. I did notice that in a working install the $guestName variable is set to null, and in the non-working install $guestName seems to be assigned some random name.
Ive tried some google-fu, but cant seem to figure out where to start on this one. All of the nextcloud files are owned by the web server user as instructed withing the nextcloud manual.