We are currently on Version 2.1.0p26 and had the same issue.
I edited the file /omd/sites/###/lib/python3/cmk/gui/background_job.py in class JobStatus to write out the job folders into a tmp file.
I found the folder/files in …/var/check_mk/background_jobs/activation_cleanup to be broken and deleted it.
After restarting the site everything was fine.