Steam problem website

I've been having the same problem. In my case I've located the cause - it's about Enhanced Steam extension and steam marketplace. If I would close a tab with market place, or would try to remove an item NOT from it's direct market page - it would cause "access denied", and the worst part - it would also lock me across all platforms: browser, client and even mobile app. It's kinda temporary but still a pain. The only solution is VPN. And the only way to not cause it for me was to train myself to not close marketplace tab - switch to anything, then close away.

As Uriah mentioned, this might be caused by Enhanced Steam or other scripts. By using them you were sending too many requests in a short time to Valve's server and therefore you were blocked. Switch IP by resetting your router or VPN.

Yeah I get this like every day. I mostly get it when opening tens of store pages at the same time or something similar. Also an Enhanced Steam user. That said I only use Enhanced Steam to sell cards, nothing else.