Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
Your script file contains bytes that are not valid text, and rush refused to read it. Here is the rule: a RushScript file is UTF-8, all of it, or nothing. That includes string literals and comments, not just code. Every byte sequence has to form a whole, proper character. What can break this? A stray byte in the middle of a character, a sequence cut off at the end, an overlong form, a surrogate, or a code point beyond the Unicode limit. Often it happens when a file was saved in an older encoding, or copied from a program that mangled the characters. The engine refuses before reading a single statement, and tells you the file, the line, and the byte position of the first bad sequence. That is much better than the old behavior, where the problem surfaced as a strange error somewhere else, or as garbled characters printed on screen. The fix is straightforward. Open the file in your editor, save it again as UTF-8, and check the spot the message points to.
Text from the Rush engine's error catalogue.