I'm sorry to hear you're struggling to sign into your TV Go account. Can I first check that you're not trying to sign in using a secondary My Virgin Media account? If this is the case, it will not work as you're only able to sign in with your primary log in details.
Also, I appreciate that your password is clearly correct if you're able to sign in via My VM, however have you tried resetting your password to see if that helps at all?
Update on how this was closed out. 2nd line support called me after I logged a fault. They 'reset' my TV Go profile and this appears to have had the intended effect. Note that no password reset or change was required. I clicked register although I was already registered and devices started logging in. Chrome and Firefox complained a bit about server request header size but after a few attempts I was in!