We had the same issues, logger a call and managed to get this resolved.
Login to your SIEM with your NGCP@domain (as username) and its password. You might fail on the first attempted but mine worked on the second attempt
Once you have logged in you can the go to the users setting and remove the @domain from the NGCP account and your back in business.
Thanks for your reply.
I couldn't get the NGCP@domain to log in at all. Raised an SR and with Support's help resolved the issue by running an update query on the users table.