Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You asked for a new process, and the machine could not give you one. Here is how a process comes to life in rush. The only thing that creates one is you, writing process new. The engine then goes through the same door that its own first green thread was born through when it started. That door can refuse for exactly two reasons, and both belong to the machine, not to your program. First, the arena is full: the single declared capacity, set in hrs.feml, has no room for another gtr's record. Second, the system could not provide randomness for the new gtr's identifier, and the engine will not invent a weak one. Notice what the engine refuses to do. It does not hand back an identifier of all zeros, and it does not give you your own identifier pretending to be a new one. It tells you plainly that the mint failed, and gives the door's own detail. Free some capacity, or check the entropy source, and try again.
Text from the Rush engine's error catalogue.