When I am doing quests and click finish quests, I get the lovely "OH NO! you don't have all the items required for this quest. But I do have the items, and I get the sP, I just get the error and have to refresh multiple times to get a fresh quest. This happens when manually placing items into my shop as well, I get "You don't have this item!", but when I refresh to my inventory, the item is gone and in said shop. This has been happening for just a couple of days now, and I use firefox.

I get those too. Using Firefox Quantum 60.0.2. ._. It usually helps to slow down a little while questing. For some reason, shop purchases tend to take anywhere from one to ten seconds to go through.

I want to say that big shops with dozens of individual items stocked are the worst culprits with the biggest delay here, but I can't confirm this accurately.