The weird site blocking could be to do with the mishmash of past companies that make up Plusnet. The accounts system possibly being on the servers of one and your connection being to another.
(Hard to explain in a few words what I mean, but basically the reason all their accounts/admin systems have been a mess for over a decade. Multiple legacy systems still in use, with bad linkages between them and ad hoc
fixes and patches applied only now and then. No overall coherent architecture).
My broadband basic info/help site - www.robertos.me.uk.
Domains, sites and mail hosting - Tsohost
Connections: OnePlus 8 Pro max 165Mbps down, 24Mbps up on Three, and B311 4G, tbb tests normally 35-45Mpbs down, 65Mbps off-peak, 9-24 up.
The EU’s multiple failures are due to a deeper malaise .... What malaise? The EU’s formidable immunity to the smallest amount of democracy. New Statesman Feb 2021.