As reported all over the place on the internet and wow forums, there is a bug with cross realm zones:

Having an auction house window open then going AFK due to being idle make the AH window unresponsive and any browse or search function will return an "internal auction error"

The solution is to close the AH window, move to exit the AFK state, talk to a new npc (or new auctioneer if possible) then come back to the auctioneer and re-open the AH or reload the UI

Unable to test all possible outcomes but this might be improved or slightly resolved after yesterday's maintenance window in game.

Closing this but will reopen if the issue becomes worse.