The issue, in simplified terms, is that the radio waves from the 5G C-band towers can interfere with the radio altimeters installed on aircraft. The radio altimeter is a critical piece of equipment for landing in instrument conditions. The concern is that the interference would cause false readings leading to a catastrophic event (i.e., crash) while on approach to land. The FAA has put out some information over the past couple days saying which airports and aircraft are the most vulnerable to system failures. They have put out a notice saying the B787 is particularly vulnerable, though I'm not sure if it's the equipment or the aircraft's shielding embedded within the composite structures. I can't help but think this is going to get uglier before it gets resolved.

Solar radiation is a different type of wave energy at a different frequency, so you can't compare the two. Solar energy has been a know issue for a long time and shielding is in place. Planes are a big Faraday cage with all the avionics and people protected inside. In addition, when you're cruising at FL380, they're likely using barometric pressure to measure altitude and not radio/radar altimeters.