Howto Wipe Data From Uninstalled Apps
Step 1
UCS stores data and information from apps at a directory: /var/lib/univention-appcenter/apps/. Chande to this directory
Step 2
Locate the directory related to the uninstalled app. Fpr Mattermost this would be /var/lib/univention-appcenter/apps/mattermost.
Step 3
Remove the directory (or move it away):
rm -rf /var/lib/univention-appcenter/apps/mattermost.
Step 4 Re-install the app
By using univention-app or through UMC.