Same problem here, pc eu server and using a controller. I've tried to investigate this phenomenon in every aspect and went to the conclusion that the pad could be the reson. To me it happens after I run on my magicka toon and the stamina bar reaches zero, or near zero %. Then I thought it could be some stamina related issue while the toon was in combat, but it seems that after you run for a while the running kinda gets stuck and it's as if you're still running even if you're not. That's why the stamina bar drains out and skills cannot be casted. Last time it occurred, I pressed the analogic a couple of times and it worked to solve the issue. I'm not 100% sure of it, but still worth a try. Still, this is a major problem, 'cause mainly I'm tank too and not being able to taunt is really a pain.