Just to make it clear, if anyone is having issues with the update to the latest version of mariadb. It's due to your database corrupting as your previous container wasn't shutdown cleanly (Something using during an update / automatic updates killing the container early). To recover your database, do the following with -all- containers that use mariadb off:
Try the `linuxserver/mariadb:version-110.4.21mariabionic` tag, let everything start up, check the logs to make sure it's all clean, then do `docker exec nextclouddb s6-svc -d /var/run/s6/services/mariadb/` (assuming your container is called nexclouddb, otherwise adjust that appropriately). Wait ~30 seconds and then stop the container and update to latest.
If this doesn't work, you will need to restore a working backup.