EU server giving the login failure message again as if I no longer have a valid account. This happened only a few weeks ago so what is going on with the system?
Everything will be alright in the end, if its not alright its not the end.
It may just be some tiny connection hiccups on your end.
I had this some time ago and was sure „My internet is great and stable. It must be ESO‘s fault!“ and when I installed a powerline adapter right next to my computer to use a shorter cable, I suddenly wasn‘t booted anymore.
Interesting, I'm also with VM in the UK. I've had occasional disconnections over the last couple of weeks and thought initially it might be the game kicking me as it coincided with others reporting problems after an update, but then I noticed it happening when I wasn't in the game. VM's automated status check tells me there's a problem at my end and they're looking into it. That resulted in several days without disconnections but it's just started up again. However, I don't have login errors, I just lose my (wired ethernet) internet connection momentarily.
If you open the Event Viewer you can see when disconnections occur. Access that from right-clicking on the Start button, then double-click on application and services log, Microsoft, Windows, scroll down to UniversalTelemetryClient. Double-click on that and Operational. Then look for Event ID 55 - if false that is a disconnection.
I'm hopefully at the point when I'll get offered a technician's appointment. We should keep each other updated on any progress - good luck!
Seems the problem "might" have been a corrupted launcher file I've been stable for the last hour since repairing files cross fingers it stays this way.