Hi,
Registration process contains 2 parts.
1) Generate registration requests and store them in DB:
`GET https://test.gluu.org/oxauth/restv1/fido/u2f/registration`
2) Finish registration. Here application loads from DB already stored registration request for validation.
`POST https://test.gluu.org/oxauth/restv1/fido/u2f/registration`
According you your registration log exception `session_expired` was happened because application can't find in DB request from step 1 by response challenge_id.
Can you share full log and get request too?
Regards,
Yuriy