All add-on pages (such as post-function configuration panels) stop loading. You just see the loading icon and message. This happens when the clock of the computer you use to connect to JIRA is off by more than 2 minutes.
This is caused by a JIRA bug (https://ecosystem.atlassian.net/browse/AC-2205).
Make sure you update your system clock (using automatic time setting through NTP is recommend) on the computer you use to connect to JIRA. If your PC is on a domain, make sure you update the time on the domain server.