ok, before doing the suggested change hibernation (and resuming) was working (at the price of (lack of splash and) luks password being asked twice)
after the change, this error message appeared early during boot: Failed to stat resume device '/dev/mapper/luks-...': No such device or directory
also, after bootup I wasn’t able to hibernate due to the corresponding menu item having vanished from the start menu: {REDACTED}
that makes me still believe without openswap you cannot hibernate
(for some reason I cannot post links, I cannot post pictures, so no screeshots, sorry)