I searched the AH for an item, clicked the cheapest one, clicked the buy button, only to find that in the time between, someone else purchased the item, giving me the message "You did not participate in this auction!".
This, I assume, is a state that Skyblocker was not prepared for, and effectively softlocked my game. I was stuck in the screen showing the item and the 'Buy' button, but neither the [<] button nor the Esc key on my keyboard would close the menu, and because I had a GUI open I couldn't open chat to 'force' another menu open (EG., "/ec") to try to close the current one forcefully.
My only option was to close Minecraft entirely to relaunch.
https://mclo.gs/m5Ht1zb1
In the log, the error happens at [13:01:30] but no actual error log was generated. At [13:01:46] I clicked the X on the Minecraft window.