Upgrading the UPM from 2.7.8 to 2.7.9 fails with the following errors in the logs.
The alternative way to install plugins works fine.
Download the desired UPM jar form Atlassian Marketplace.
Go to this URL on your Confluence site:
Click Choose File, find the UPM plugin JAR and upload the JAR into Confluence.
More info for other products in Updating UPM by file replacement