What can possibly clog filters on booster? Not like LOX get to tank unfiltered. Some form of ice, but what can from ice there except LOX itself. Any water and CO2 should be purged before fueling by nitrogen.
That's the big unsolved question. Easiest answer is that they are using preburner exhaust to pressurize the tanks, but putting CO₂, CO, water vapor and maybe a trace of unburned methane into an oxygen tank sounds like such a bad idea - but like you, I can't think of anything else that could block the filters.
If water ice floats, wouldn't it not clog the lines until you run out of propellant, in which case the clog is moot?
I was under impression that they pressurize tanks by pure oxygen
That was my impression as well. I wish I could figure out where people are getting the idea that they are using combustion exhaust to pressurize the tanks. Seems like a weird idea to me.
Easiest answer is that they are using preburner exhaust to pressurize the tanks
Has there been any evidence that they are doing this? I've seen this mentioned in a number of places, but it seems mostly like speculation.
That said, I can't think of any other source of filter clogs, unless one of the workers left their hat or something inside. Perhaps Tim Dodd can clarify this during his upcoming Starbase tour.