Confluence 3.5 comes with UPM 1.2.4 as default. Upgrading UPM to 1.3.0 or 1.3.1 does not help. So far there is nothing written in the log I can see, it just keeps spinning (in Install or Upgrade tab where it needs to contact PAC).
Install Confuence 3.5
Set a proxy with authentication (in my case I used CCproxy). For example, here what I have in setenv.sh:
Try access UPM and check on Install or Upgrade tab, it will just spin indefinitely.
jiraissue macro works fine: