Thanks Milan, turns out our generated passphrase is exactly 64 bytes long.
If your passphrase is longer than 63 characters, please read
https://gitlab.com/cryptsetup/cryptsetup/-/issues/627
(And try 5.11.4 kernel.)
If not, please report this upstream (or to bugzilla) with more info.
Milan
--
Ian Laurie
ilaurie(a)bigpond.net.au