Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
Your script tried to read an uploaded form, but the boundary it passed is not usable. Here is how it works. A multipart form is a body made of several parts, separated by a marker called the boundary. The client picks the boundary, and announces it in the content type header, in a field called boundary. To read the body, rush needs that exact marker. Notice the two sides of responsibility. The body belongs to the client. If the body is broken, rush does not raise an error, it answers null, because outside text must never be able to end your script. But the boundary is handed in by your script, and that is your side. It is refused if it is empty, perhaps because the header had no boundary field. It is refused if it is longer than seventy bytes, the limit of the standard. And it is refused if it contains a line end. Without a proper boundary there is nothing to split the body by, and rush will not guess one. Read it from the header correctly, and pass it on.
Text from the Rush engine's error catalogue.