Debugging login issues after upgrade and backup / restore

[Alec Walker]

I have a login problem after the following activity:

1. 'Backup system' used to export existing projects from JIRA 7.1.2 (running on Linux). This instance uses a Crowd 2.8.4 instance on a separate virtual server;
2. Build new JIRA Software 7.1.2 on a temporary server (Windows), restore from backup created in (1). This instance uses a separate Crowd 4.0.0 instance (Windows). All works OK, I can log in and see projects / issues
3. Upgrade this temporary server to 8.5.4. This worked OK, I can continue to login and see projects/issues
4. Backup system on temporary server, move backup file to new 8.5.4 final server (Windows), import backup. This instance is configured to use the same Crowd 4.0.0 instance as in (2) and (3). Restore appears to go OK, all Crowd Directory tests pass.
5. Problem: I can no longer log in to this new 8.5.4 instance with the user account (with admin privileges) in Crowd 4.0.0. 
   1. The error message I get is: "Sorry, an error occurred trying to log you in - please try again.". This is not helpful...
6. I can log in with a JIRA-internal Directory 'admin' account that I've created but this cannot see any projects and does not have any administrative permission despite me adding it to the 
7. I can log in with the recovery admin user and see evidence of the restore, i.e. all of the projects listed 
8. I have removed, re-added and re-tested the link to the Crowd server; all appeared to go OK.
9. I have re-indexed the new instance, appeared to go OK.
10. I have tried enabling additional logs via the log4j.properties file and whilst these changes (adding 'DEBUG' to multiple rows related to security) all appear to come through into the profiling and logging page when I've next started in recovery admin they fail to generate any new log files. 

Anyone have any suggestions as to how I can proceed? I'm stuck without any useful error message.