just in case someone faces into the same problem.
After an update of OMV yesterday to 3.0.71, today I saw the above named error message, while opening the rsync tab.
The solution, I found here in the forum, to delete the section <jobs> from config.xml and lose all my rsync jobs wasn't my preferred solution.
I found out that there was a mistake in the config.xml for every old rsync job (created with OMV 2).
Within the <dest> or <src> section the folder uuid was in the wrong field.
As an example:
Maybe this this will help someone.