That was a good approach. Because it reminded me that I need to check the possible adjustments to distributed monitoring on remote sites. (I had always forgotten about this before).
I changed the log levels on all remote sites and set them back to standard and then the test upgrade went through without failing.
I may have messed something up with the distributed notifications, but I can fix that.
(A small question in between, when doing distributed notifications, should I set up the connection from the control center to the remote sites or rather in the other direction? )