it's both, and I haven't changed anything on either computer recently, the only change at my end is that my subscription recently ended (was a 2 month one) and I resubscribed, it started happening immediately afterwards but there were also patches being installed at that time. Because the issue is only with my subscriber account regardless of which computer I use, I assume that the issue is most likely with the account. Also if you look at the first post I made you'll notice that I said I "normally" play on the same computer, so I don't think that really "clouded" anything I said.
If you look back through this thread you will see that there are a few other people complaining about this, it's still possible to play the game of course, just an irritating inconvenience.
I opened a ticket in game. The customer service reps got back to me quickly enough - never had a problem there, customer service has always been good in my experience. They apologised for the inconvenience, told me they were investigating and advised me to post here, so I'm posting here.