I have to confess I cannot understand what is happening here. You should receive an activation code which you have to use only once and when you have input that, you have access to the self-assessment online tax return completion software. Even if the umbrella company completed your returns for you in the past, that should not stop you registering yourself.
The number you were advised to call has been blanked out by just answer again. I don't know why that is because it isn't a private number. I shall take that up with them.
What I suggest you do first is read the notes here
. Before you do anything else, call HMRC online services helpdesk on the number here
and tell them what you have told me and give them the details you just gave me in your last post, ie the reference number starting 4JC7 and the details of the email you received. They should be able to tell you what the problem appears to be and to rectify it. If they can rectify it, it may take several days but you can ask them to call you to tell you when the problem whatever it is has been fixed. Ask them if you can get a new activation code online as it says you may be able to in the first link I gave you.
As you say, it might be worth the expense of taking all your tax return figures to a local accounting firm (ask friends for a recommendation) and have the return completed and submitted by them. It's a fairly quiet period right now and it could all be over by tomorrow lunchtime and you can relax at the weekend.
Whatever you decide, make a complaint to HMRC. You can read about how to go about that here
I hope this helps but let me know if you have any further questions.