I recently rebuilt the top end of my FS450c and had a screen that looked awfully similar. Pulled, cleaned, new filter and new oil. Haven't run it since as it was late in the day and I just haven't gotten back to the bike. Like you, I'm hoping it's a first run after rebuild issue that does not persist. I'm a bit more concerned since it was just a top end rebuild and not new piston, rings, etc. I'll update as soon as I get her running again and recheck the oil, screen and filter. Fingers crossed, for both of us!