I performed the upgrade from Ubuntu (server) 16.04 LTS to Ubuntu 18.04 LTS pretty late this time (on December 2019: it was about time!) and it mostly went smoothly except for a few bugs that I solved as described below:

• no internet after upgrade: the weirdest bug was the fact that I lost most internet access from server (not to server) after the upgrade. I solved this issue as documented in this thread: I edited the file /etc/systemd/resolved.conf to add the line:

  DNS=8.8.8.8
          

  and re-started the system with

  systemctl restart networking
          

• php reconfiguration: php was not working anymore because the previous version was php5, having become obsolete. I re-installed the base php packages:

  sudo apt install php libapache2-mod-php php-mcrypt php-mysql
          

  and enabled the php module in apache

  a2enmod php7.2
  systemctl reload apache2
  	

• bug in nextcloud: I was on an old version of nextcloud, which is not upgraded by the system ugrade since it is not a distribution package. Nextcloud showed a blank pages and apache logs (/var/log/apache2/error.log for me) gave the following message:

  Local::copyFromStorage() not compatible with OC\Files\Storage\Common::copyFromStorage
          

  The bug was documented on nextcloud github repository and I solved it as was done by one of the developer in this commit by editing the file lib/private/Files/Storage/Local.php in nextcloud directory to add the argument $preserveMtime = false to the function copyFromStorage.