This is a continuation of JSWSERVER-11775 where the majority of the timeouts appear to be prevented JIRA Server 7.2.7 and there is a new user friendly message created for users.

However, there still appears to be some scenarios where this timeout is hit.

With release of 7.2.7 (server) we will change the behaviour of the quick release window (a bit).
If the release process takes longer than expected and the initial request times out - it will result with a new and more informative message:

The link provided ("1 issue" in the example above) should open the issue navigator filtered to a release version so that one can see if all tasks have already been processed properly.
Due to the nature of the release process in some extreme edge cases it MAY happen that the release takes more than just "few" seconds. During this time - it may happen that not all tasks will "vanish" from initial Kanban Board or the released list (from link provided) may look incomplete. The process - though - will continue in the background and all tasks will get processed and properly marked in the system, eventually.
The team is aware of this situation and we have already focused on performance issues around the release cycle.
For majority of situations, though, the process should not take more than "few" seconds and simply "reloading the board" as the new message suggests should show final state of the board.
I hope this will ease the confusion mentioned in the OP at least a bit

This is being opened as an attempt to track these extreme edge cases that MAY happen where the release takes more than just "few" seconds, triggering the timeout.