so it turns out that my earlier problems with the Lychee gallery upgrade from v3 to v4 as described in https://github.com/LycheeOrg/Lychee/issues/1527 were due to that VM being kinda old and creaky and therefore having 32 bit php. After creating a new VM and lots of own mistakes along the way, things are now running again and the old links to pictures are being correctly redirected to new URIs. There is a lot of stuff I don’t know about modern php, it turns out.