This happened today and yesterday. I accepted a quest, today I had to buy 2 items. I left the page, bought both items, and went back. It said I did not have those items. I bought them again, made sure I had them in my inventory, turned them in but I got an error saying I was not on a quest. I was on 3/15 quests the entire time.

Happens with a lot of quests. I think sometimes it's a caching issue? When I get told I don't have the items, I just refresh the page to really make sure.