Solved.
In etc/default/grub the line GRUB_CMDLINE_LINUX_DEFAULT=… must contain the identifier as a device, not as a UUID. Even if that device has that UUID.
With the data from my OP:
I replaced
resume=/dev/disk/by-uuid/33333333-3333-3333-3333-333333333333
with
resume=/dev/mapper/crypt_sda2 and it works. 
Probably (?) because the UUID of /dev/mapper/crypt_sda2 (from blkid) points to all btrfs subvolumes (in fstab), and even if /swap would be correctly identified, the physical offset could be incorrect.
An equally lucky sunday to all you folks!
Thanks for the readiness to help!