This error often occurs, when the steam trading server is under heavy load. In example this happens right before important e-sports matches because players bet a lot of items on these games. In order to do this they have to trade via trade-offers with the betting systems.
The heavy server load makes your request (loading your trade offers) taking more time and after some seconds, if the load is not finished, a timeout occurs. This timeout results insuch errors.
The same problem also happens often to me, when i try to load my inventory with 3500+ items. It takes pretty long and if the server is too busy, I often get a timeout.