Hello there, I had an ‘enjoyable’ evening updating my server to 18.04 as well. The main issue for me was the updated php. I went in and reinstalled all the php modules as per the admin doc.
Also this post helped as well: New Nextcloud Server Incorrect PHP Version
I also reinput the php.ini settings in as many php.ini files as I could find - I think there were about 3 or 4!
Finally a reboot or 2 along the way got it going.
I feel your pain! I managed to re-enter Earth’s atmosphere, but more by luck than judgement!
Good luck