Space Wizard fails after deactivating server app with space blueprint

Issue Summary

Space Wizard fails with "An unknown server error has occurred" after deactivating server app with space blueprint

Steps to Reproduce (You may need to shift-reload the page before launching the space wizard)

1. Have an add-on installed that provides at least on space blueprint
2. Deactivate this add-on
3. Try to create a new space with the space wizard (Spaces -> Create Space)

Expected Results

The wizard shows the space blueprints to select from.

Actual Results

The space wizard shows "An unknown server error has occurred"

The logfile shows this:

[INFO] [talledLocalContainer] ..... WARN [http-nio-1990-exec-7] [confluence.impl.hibernate.ConfluenceHibernateTransactionManager] doRollback Performing rollback. Transactions:
[INFO] [talledLocalContainer] ->[PluginReadWriteTx]: PROPAGATION_REQUIRED,ISOLATION_DEFAULT (Session #.....)
[INFO] [talledLocalContainer] – referer: ...../confluence/pages/viewpage.action?spaceKey=PP&title=x | url: /confluence/rest/create-dialog/1.0/space-blueprint/dialog/web-items | traceId: ..... | userName: .....

Checking the dev console logs:
The call GET /rest/create-dialog/1.0/space-blueprint/dialog/web-items results in

{"errorCode":500,"errorType":"UNKNOWN"}

Notes

This problem occurs on Confluence 6.11.2 and above (although we did not test every single released version of Confluence), but not on 6.11.1 and below (again, we tested some 6.x and 5.10.9 which did not show the problem). The release notes to 6.11.2 seem not to relate to this issue ...

If the add-on is reactivated, everything works fine again.

The problem shows on MySQL, PostgreSQL, and H2. It occurs in the Atlassian SDK as well as in production.

Workaround

The workaround mentioned in https://community.atlassian.com/t5/Confluence-questions/Confluence-add-Space-fails-An-unknown-server-error-has-occurred/qaq-p/893077 fixes the problem. In fact, dropping the AO_54C900_SPACE_BLUEPRINT_AO after server stop, removing the plugin cache directories, and restarting the server also fixes the problem: new spaces can again be created.

DROP TABLE AO_54C900_SPACE_BLUEPRINT_AO;

There is a minor difference in the table between the 6.11.1 and 6.14.0: The name column contains label keys instead of labels in the NAME column. But replacing these with arbitrary strings did not fix the problem. Even if the server is restarted and the plugin cache dirs are cleared.