[CCMA] Accessing the Migration Assistant inside Confluence gives error msg "We can't communicate with your server"

Issue Summary

When customer tries to open the Confluence Migration Assistant, he gets error message in a pop-up:

We can't communicate with your server

Customer must have a list of Migration attempts beforehand. The issue only happens after having several test attempts visible in the Migrations dashboard

Steps to Reproduce

1. Go to Confluence Administration Menu
2. Click on Migration Assistant
3. Considering there were migrations attempts already made, the Migration Assistant will display the Migrations dashboard first thing. (empty CCMA takes you to the CCMA Home screen)
4. Wait for it to try loading the screen/table view until you get the pop-up error message
5. Additional behaviour that can happen is the never ending load of the Migration dashboard or just partial loading with the load spinning also happening

Expected Results

The Migrations dashboard screen should open normally

Actual Results

The Migrations dashboard screen either tries to load the list of migration attempts indefinitely or it loads partial information and stays attempting to load the rest indefinitely as well

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

2020-11-04 23:46:40,712 ERROR [http-nio-8090-exec-1043] [common.error.jersey.ThrowableExceptionMapper] toResponse Uncaught exception thrown by REST service: class java.util.HashSet cannot be cast to class java.util.List (java.util.HashSet and java.util.List are in module java.base of loader 'bootstrap')
 -- referer: http://confluence.mycompany.com:8090/admin/migration.action | url: /rest/migration/latest/plan | traceId: f7c251d78569b440 | userName: admin@mycompany.com
java.lang.ClassCastException: class java.util.HashSet cannot be cast to class java.util.List (java.util.HashSet and java.util.List are in module java.base of loader 'bootstrap')
	at com.atlassian.migration.agent.service.check.space.SpaceConflictChecker.retrieveConflictingSpaces(SpaceConflictChecker.java:71)

Workaround

Currently, the only alternative available once CCMA breaks like that, is to reset it to "factory default". The KB informing this alternative can be found below. Of course, this is not a workaround customers are looking for and a bug fix must be in place as soon as possible

• Restore the Cloud Migration Assistant for Confluence to the default version