I have had this error on my account since December. My bank account has changed so my direct debit isn't working. Plusnet are email and texting me to tell me to log into my account and sort payment but I CANT GET PAST THIS ERROR MESSAGE Grrrr!
I can't get in touch with Plus net by phone as it is permanently on hold.
I am in a similar position to yourself. I get "An error has occurred". "You cannot make any change to your account while there is a phone order in progress." I have no phone order in progress. Hence I cannot change onto a different package, nor look at upgrades "My Perks" etc. Like you, I cannot get in touch with PlusNet at all.
PS: I also keep getting a message saying I have not changed my contact preferences for over 6 months - and I have.