Check the notes here : 0x800701b1 during launch (Update: It has returned) - Community Support / Install, Performance & Graphics - Microsoft Flight Simulator Forums
“I was finally able to work around it on Sunday by turning off my wifi during the initial xbox game services UI splash screen and then enabling it once the actual game launched. However, I’d really like to have it fully fixed and not have to do these extra steps. (This also worked for the Forza users above)”