This seems to be a new bug not mentioned in H&S that is happening to myself, players on IRC, and other friends in the game.
Occasionally you will attempt to buy any type of item in the store (I'm personally trying to buy runes), and it will pop this error code. It will not take your IP, nor will you get the item.
I purchased 6 runes successfully
I tried purchasing a 7th unsuccessfully 10 times.
I left it alone for 5 minutes.
I purchase 1 rune successfully.
I tried purchasing another unsuccessfully several times.
I left it alone for a minute.
I purchased 4 runes successfully.
I tried purchasing another, unsuccessfully.
Etc. It seems that once you encounter the error, give it a rest for a sec. This may still just be related to the store being overloaded with too many purchases at any given time.
EDIT: Oops, I hadn't finished the title.
Just popping this up to the top, I experienced this same issue. Like you said, it seems to come and go at random. After a few tries, it successfully allowed me to purchase.
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