Seems today I cannot trade with anyone from dispenser. Either error 28 or 11. Eventually 2 if I try too many times.
Tried the Steam client and website trade. same problems.

"There was an error accepting this trade offer. Please try again later. (28)"
example
https://steamcommunity.com/tradeoffer/202471542/

Twelve hours later, things seem to be working properly again. Not sure what has changed.