Safer fix for the above identified issue, kudos to @vpeter for the tip, no need to change language or delete any files.
Paste the following commands into SSH if you have the above issue.
systemctl stop kodi
/usr/bin/sqlite3 /storage/.kodi/userdata/Database/Addons27.db "UPDATE addons SET metadata = REPLACE(metadata,'https://addons.coreelec.org/9.0/','https://addons.coreelec.org/9.1/');"
/usr/bin/sqlite3 /storage/.kodi/userdata/Database/Addons27.db "UPDATE addons SET metadata = REPLACE(metadata,'https://addons.coreelec.org/9.0.1/','https://addons.coreelec.org/9.1/');"
systemctl start kodi