Course copy failing when including user data, now blocking further course copy

Hi all, first of all thank you for this great forum and all the support available here.

I am trying to copy a course on my site. Copying the course without including enrolments worked fine. However, including enrolments and user data, I get an error on running cron:

Execute adhoc task: core\task\asynchronous_copy_task
... started 05:47:03. Current memory use 44.4MB.
Course copy: Processing asynchronous course copy for course id: 4
Course copy: Can not load backup controller for copy, marking job as failed

This is now impeding me from copying further courses, even without including enrolments. On /backup/copyprogress.php?id=<coursenr>, there are now three courses:

Course copy 1, started 7 dec. Current operation: Restore. Status: 86.39%

Course copy 2, started 8 dec. Current operation: Backup. Status: Process pending

Course copy 3, started 9 dec. Current operation: Backup. Status: Process pending

Course 1 and 2 include enrolments. Course 3 does not include enrolments. It seems as though Course 1 is blocking further copying of courses each time I run cron.  Cron is not set up to run automatically so I run it manually after initiating course copy. I have cleared caches several times, before and after running cron. I have tried rerunning cron both with and without asynchronous backups enabled.