SD Fair enough.
I posted the link to show it was the genuine thread URL, and the archive one for comparison. The ARRSE link showed that there was a thread asking to see what happened if people followed a PPRuNe link - most of them were prompted to log in or register.
This is my theory....
I don't know how ISP's assign IP addresses, but I assume that dynamic addressing is used, so people may get different IPs assigned to them each time they access the internet. If that IP has already been used to log in, they will be blocked. If not, a link will take them to the thread.
Simple really, but it had me stumped for a few days. I'm not qualified in or work in IT.
It is unfortunate that misusers have made these (hopefully temporary) restrictions necessary, but such is life.