Linuxserver/Mastodon:4.0.2 Create account - 422 Request cannot be processed

I have the container running and talking to Postgres. DB is created as expected. Redis seems to be working. All running on the internal network.
Going to the Create Account screen and entering appropriate details results in 422 where it seems the request was understood but cannot be processed.

This is what was posted:
authenticity_token=VVYAQMfN1kz_FXG7LEzwhCh… lots more stuff
user[account_attributes][display_name]=MyDisplayName
user[account_attributes][username]=MyUserName
user[email]=me@mydomain.com.au
user[password]=C4rc3oCryptographicallySoundPwd
user[password_confirmation]=C4rc3oCryptographicallySoundPwd
user[confirm_password]
user[website]
accept
user[invite_code]
user[agreement]=0
user[agreement]=1
button

Here’s what the container log has:
2023-01-16T00:44:02.796Z pid=192 tid=2ga4 class=Scheduler::SuspendedUserCleanupScheduler jid=1d345ae8bee6bc83ae8d47eb elapsed=0.012 INFO: done
2023-01-16T00:44:02.798Z pid=192 tid=2g74 class=Scheduler::AccountsStatusesCleanupScheduler jid=553c7cd6a7d2133df910d491 INFO: start
2023-01-16T00:44:02.829Z pid=192 tid=2g74 class=Scheduler::AccountsStatusesCleanupScheduler jid=553c7cd6a7d2133df910d491 elapsed=0.031 INFO: done
[55bebffd-5b40-442b-8818-9460ef73d0a5] method=POST path=/auth format=html controller=Auth::RegistrationsController action=create status=422 duration=10.55 view=7.58 db=0.00
2023-01-16T00:45:02.974Z pid=192 tid=2g7o INFO: queueing Scheduler::SuspendedUserCleanupScheduler (suspended_user_cleanup_scheduler)
2023-01-16T00:45:02.977Z pid=192 tid=2fts INFO: queueing Scheduler::AccountsStatusesCleanupScheduler (accounts_statuses_cleanup_scheduler)

I guess my question is: where should I look for further details on what the issue is with processing this request? I’m at the point of guessing what may be wrong here and the Mastodon doc is pretty vague, but it implies there might be some more detailed logs somewhere. However I note the constraint of not wanting to tell the user too much lest they figure out what security is happening and a generic 422 response certainly fulfills that requirement.
This container is running on ARM64 and as such it is one of the few that does.
Any help most appreciated. Thanks in advance.