Can't upgrade directly to version 6.6 if Bamboo is connected to Crowd

Summary

Upgrade will fail with IllegalStateException in upgrade task 60201 if upgrading from version 6.1 or lower.

Steps to Reproduce

1. Create a fresh Bamboo 5.12.5 instance
2. Connect it to Crowd for user management
3. Create two projects
4. Shutdown
5. Upgrade to Bamboo 6.6

Expected Results

Successful upgrade

Actual Results

The below exception is thrown in the atlassian-bamboo.log file:

2018-06-28 10:27:50,471 FATAL [localhost-startStop-1] [UpgradeLauncher] Upgrade task error: Task for build 60201 failed with exception: java.lang.IllegalStateException: Connection pool shut down

Workaround

Upgrade to 6.5.x first and then to 6.6.0