Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You were waiting for a reply, and when rush went to read it, something was wrong with the mailbox. Here is how replies work. When you send a letter that expects an answer, the delivery service eventually drops the reply into your own mailbox, which is a part of your own heap. Your message call then looks in that mailbox for a letter that says it answers yours, matching by the letter's id. That look never leaves your own memory, so it should be safe and quick. This error means the look could not complete honestly. Either the id being searched for is not a real id, or the reply that the mailbox points to can no longer be read from your heap. Rush could return some invented value to make the wait end, but a fake answer is worse than none. So it stops, naming the letter in the first slot. If you see this, check that you did not alter the id you got back, and that the sending thread is still alive and has memory left. Then send the letter again.
Text from the Rush engine's error catalogue.