i configured different yubikeys (5 NFC, 5C nano and 5 nano) running Windows 10 and firefox 101 and the login via webauthn works here …
It does not work running firefox (fedora 35) - i get this error message:
“Validation failed: An attempt was made to an object that is not, or is no longer, usable”
It works running chromium 102 (fedora 35), so it seems not to be a fedora issue.
Running firefox (fedora 35), i can use webauthn with bitwarden and nextcloud, so it seems not to be a firefox issue.
IMHO, it is a problem with mailcow / firefox / linux problem.
Can anyone confirm this?