I’ve noticed issue. You can create accounts, but you can’t log back into them.
For testing go to to create an account:
You’ll get an email that your account was created and you’re logged in, but if you go back to try to log in you can’t get in.
Correct password gets you back to the log in, but incorrect gets you an error message.
You can visit the WHMCS install to verify your login is good:
I don’t believe it to be a WHMCS issue, but I can get you an admin account if needed, otherwise I turned on support access in wp.