Similar issue occurred for us, which I figured out, posting here as this might help someone.
Whilst investigating a Job, I noticed that a folder-name is being created with incorrect name ([Task does not exist or is not selected.]), which (although was causing issues (as the folder name consists special characters) which caused the process to fail.
After investigating I found out that a Job-variable was being assigned its value using a non-existent task's ID (I guess it was a copy-paste from another job which perhaps no longer existed). The folder name needed to be suffixed be the job execution date/time. To fix this, I changed the ID reference to the ID of the current Job and voila, no more [Task does not exist or is not selected.] folder-name.