Activities/Events Fail to Load. Spins forever

Bug Reporting Hub Crashes (CTDs)
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3

I think this may be similar to the issues posted here:

Can’t start a flight, just get a loading circle when clicking ‘start flight’ - Bug Reporting Hub / Install, Performance & Graphics - Microsoft Flight Simulator Forums

Long loading/infinite time for free flight after pressing “Start flight” - LOD reduce as workaround - Bug Reporting Hub / Install, Performance & Graphics - Microsoft Flight Simulator Forums

Reducing the LOD from 200 to 100 let me successfully load into the challenges, so you may want to try the same to see if it helps with other activities as well.