Attended a customers premise the other day where they were unable to perform offline servicing of one of there images. After trying to manually delete the files etc, learned that it was a mounted WIM, and found this post. This resolved the issue:
Can’t Delete ConfigMgr_OfflineImageServicing Folder After Failed Servicing.
However, I did not need to reboot the server. Followed the comment and used the
dism /unmount-win /discard
first. Then was able to remove the folder. Started the update again, and it went through without a hitch!