It is one of the most frustrating experiences I've had with tickets. Had tickets in basket for both DK-Russia and DK-Wales, but got thrown out for not finishing order in time, because the system was just stuck on loading when trying to proceed to next page.
Spent 6 hours trying to buy tickets Tuesday, with constant errors. Had a meeting at work from 10-11 yesterday and apparently the issues got worked out for a short period from 10:30-11 where the leftover tickets got snatched. Pretty much the only time period I couldn't monitor the situation and there was no warning from UEFA that it was "fixed".
Just imagine how fecked they would be if they had 3 times the traffic or whatever it would be with full capacity. Shocking system, never experienced anything like it.