Cups reports conflicts where there are none

I have used Manjaro for many years, but after some problems occured which I couldn’t solve, i tried a new installation. I had been using my printer in a way that I set up various printers for the various tasks, like A4 or A3 or plain or photo or duplex printing. This way I have all the settings done for the work I do. In total there are at the end 6 different printers, but they print all to the same printer of course.
Now I wanted to set this up in the same way as I had it before and get continuously conflict messages. E.g. it says that “A3” and “tray 1” as paper source conflict with each other, but it doesn’t work either when I change it to “tray 2” as paper source. I didn’t try the automatic option, since I fear it will pull in paper from the wrong tray (where there is no suitable paper available). The only way to solve is would be to go back to “A4” paper size, but that is useless since I need the selection for A3 printing (and it doesn’t make sense if the driver provides the option for A3 paper size, but at the end it isn’t accepted). Another is the choice of duplex printing on A4-paper, it again says that there is a conflict between paper size A4 and duplex print, which is nonsense. I’ve done it a thousand times on the old installation.
As said, I had this working before, but since it didn’t cause any trouble for many years, I don’t remember what I did then in order to get it right.

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