Go to Marketplace and put anything starting with a [ (left square bracket) between quotation marks in the search field. For example for you farming and production system we would often search for "[G&S]". The same error does not happen when the bracket is not the first character. E.g. "Bread [G&S]" will not trigger the error, but something like table AND [G&S] would.

Also MP always says: "Second Life Marketplace encountered an unexpected error processing your request.

We've been notified about this issue and we'll take a look at it shortly." Shortly must be years in this case.