LDAP browser tool helped a bit and allowed me to see a more specific error:
[LDAP: error code 49 - 80090308: LdapErr: DSID-0C0903A9, comment: AcceptSecurityContext error, data 52e, v1db1]
After a bit of research I've managed to connect using account@domain.co.uk format in "Bind credentials" username.
This might be worth adding to the pfSense-LDAP troubleshooting guide.