CRA told me this morning that there are many changes going on with this process.  The normal RZxxxx for my business number does not work when I file for clients' T4s.  I was told to use my RPxxxx number but I had to get a new WAC for that number.  

I tried that but it does not work either and I get the error messages spoken about in this stream.  There has been zero communication from CRA regarding any of the changes that they are doing and, to top it off, their website is breaking down with no one answering the phone.

I might try, as suggested, using the client's PRxxxx number but the question I would have would be will my WAC work using their number.  I cannot always go online to the client's CRA account because I do not have access to all of them but I can (or used to) still be able to file their T4 slips.  I hope I can, anyway.