ERROR Error processing element InMemoryAppender ([Appenders: null]): CLASS_NOT_FOUND ERROR Unable to locate appender "InMemoryAppender" for logger config "root"
I get this with starting Unifi service after I upgraded Ubuntu 16.04.7 LTS --> Ubuntu 20.04.2 LTS...
I see https://community.ui.com/questions/UniFi-windows-service/c529d311-1667-4495-8010-f213eeac7651#answer/693b2b20-a51e-4d53-a4d1-ea5a57416711 explains this is a bug in the release and you must downgrade to 6.0.x.
For me this meant removing Unifi because after trying to install a 6.0.x version it said I couldn't upgrade to 6.1.71.
Before getting to that point I was supposed to have taken a backup https://help.ui.com/hc/en-us/articles/204952144. So if you're doing this yourself, probably start there.
Following https://help.ui.com/hc/en-us/articles/360015373734-UniFi-How-to-Uninstall-the-UniFi-Network-Controller I ran:
dpkg -P unifi
# I had also installed unifi via apt at one point, so just to be sure I ran
apt-get --purge remove unifi
Then, I went to https://community.ui.com/releases and searched for the latest 6.0.x release of "UniFi Network Controller", which took me to 6.0.45 https://community.ui.com/releases/UniFi-Network-Controller-6-0-45/8d3b98e1-b9d4-4ab3-b8da-721dbe9ab842. On that page there's a link to download the deb file under "Download links". I downloaded and installed the deb:
dpkg -i unifi_sysvinit_all.deb
At this point, the site http://localhost:8080 didn't work because service unifi status showed this error:
WARN Unable to load properties from '/usr/lib/unifi/data/system.properties' - /usr/lib/unifi/data/system.properties (No such file or d
So, I restarted the service:
service unifi restart
And, now going to http://localhost:8080 shows the controller.