Thanks. This was it. My last attempt had not activated the recovery mode. This time, it took a couple of hours to convince the system to boot in Recovery mode. It looked dead until I decided not to touch it for an hour.
May be this info is useful for others: SIP is responsible for some odd effects. E.g., all of a sudden, I was not able to save on a server or create a folder. I am the Admin of that server, and all actions went well on the second drive of it but not on the boot drive. I also had issues with access from my PB to my main computer. But it did work the other way round. But both machines have the same OS and the same Admin.
The issue with no access to Mail attachments in pdf is also odd because I had access to other attachments. Two apps would show the content of a pdf - file, I was able to copy that and find the original. Only opening had been disabled.
Thanks again, also because this advice was not found after some weeks of search.