After I wrote the last entry, I have installed the VMS updates that I had downloaded before. Of course, after backing up the system disk.
Rebooted once - and, as expected, all processes restarted. Like a charm.
Second, I have changed some system parameters: The number of processes has been cut down to about half - the maximal number of processes running on the system is about 60, so keeping over 200 slots is, well, slightly overdone :). I still have to check the T4 statistics, but the impression is it did help.
Still to be done: clean-up and re-organization of the system disk. Backup took a LONG time, there are quite some files that are really big: the web backups, for instance, take a lot of space - and time. These files can be very badly fragmented : I got one that was split into over 7000 fragments….
The webs - kept on my workstation - have been completely written to DVD as an archive. So a real backup of the PUBLIC web is not relly needed: most files are rather static.