Authentication Failure

Hello Sir,
Not sure if this should be in the web protection or management forums. Going to try here, as authentication servers pass.

I'm using 9.004-33, AD SSO standard proxy configured. Things had been working fine until I needed to replace my AD DC yesterday. Since then, any web access is met with an 'Authentication Failure' web page from Sophos. Looking in the live web log, I see the following message:

sys="SecureWeb" sub="http" request="0x80181de01 (this changes for each line) function="adir_auth_process_negotiate" file="auth_adir.c" line="1076" message="gss_accept_sec_context: An unsupported mechanism was requestedNo error"

I've removed and re-added my authentication servers, and tests run there pass successfully. I've also rejoined my Sophos machine to the domain, that went fine too.

Any pointers here? Clients which function as transparent clients work fine.

Thanks,
Tim