I had a semi related situation recently. Mine turned out to be server overload and an eventual burnout. My server was slowly failing and it was somehow not fully generating the ubb pages. My members had to hit RELOAD to see pages when they got a blank screen. Theyd often post a reply and then the screen would go white. They hit reload and they were taken to the page they expected.
Based on that you might want to check your server load (if you can). You will most likely get the errors during peak usage.
I also got these errors more when enabling the SPIDER FRIENDLY option. I have since disbaled that
