I see this message for more than 24 hours. I can't get items from the bot or put items in. Steam service is not down because:
1. People still can buy items from the bot and they do
2. I trade manual with my contact list and trading works great

Others VIP shops put/get items, I can see it checking their history.

Thank you in advance for help.

I found a reason. It's about one dota2 key in the bot inventory. If it's involved into an exchange process dispenser will show the error. This key can be easily viewed in the bot Steam inventory or in Dispenser UI. So the key exists, it simply crashes on exchange.

Joto, please, figure it out.