As above. I solved it by uninstalling the agent completely and
downloading the full agent from the server and installing it.

There was an other interesting thing to notice, despite of the server
running fully under ZCM10SP3, the files to download were not all
updated to 10.3 . I had to reapply the update to the server, to get all
to the correct version.

I have the impression, that the update scripts are far away from being
robust, they seem to be extremely fragile to any unforeseen happenings
and settings they encounter.
W. Prindl