many thanks for your ideas!
First I tried your first suggestion: I commented out the line in "/etc/crypttab" and copied all entries of "/run/systemd/generator" to "/etc/systemd/system". No success.
But your third suggestion was a hit! I added the option "initrd" in "/etc/crypttab"
and run "mkinitrd" as root in the console. The password prompt appears earlier now (before systemd starts as you told). After the first restart I waited more than 5min - no timeout!
cr_sdb3 /dev/disk/by-id/ata-WDC_WD2500BEVS-60UST0_WD-WXE808P0A960-part3 none initrd
So, for me this issue is solved now. Nevertheless I will file a bug report to openSUSE, because of ignoring the timeout parameter in "/etc/crypttab".
Thanks to all!