After one of the latest updates, a big square box appears at the center of whatever I have selected, either ship or asteroid.

I don't use any custom skins, but I do use targetless which rearranges the HUD, however, this only began to happen after one of the last updates, didn't make note of which one.

This happens on both phone and tablet, both Android.

TCS/MultiAim will do that if it can't find the image files for the crosshairs.

Tried it without plugins yet?

Yes, you are correct. It is TCS>MultiAim at fault.
It does appear that the stand-alone MultiAim works just fine.

Thanks.