I have tried uninstalling and downloading twice now, but it doesn't change anything. I was really excited to play H2, so Ill keep giving it a shot until something works. Specs are AMD Ryzen 2600X CPU and Radeon 5700 GPU. I haven't had problems with the base version of the game, or any of the previous Steam flights, and from looking online, it doesn't seem to be a common occurence. Hopefully we can expect an update very soon that can help resolve this issue.
I have just fixed it!
It was caused by a Windows 10 privacy setting. I recently got a Windows Mixed Reality headset (need it for work related things) and it was trying to sync with that I think. However it may be syncing with another device in your case.
Got to Windows 10 settings > Privacy > Other Devices and disabeld the "Communicate with unpaired devices" checkbox.
After I did that and started flight again this "Gaming UI" pop-up about syncing data actually showed up in-game and asked me to try again or used the last save data. Try again obviously didn't work because I turned off the setting, but clicking "use last save data" caused it to begin re-syncing and after that reached 100% I was finally able to enter the menu.
If this does not work for you try and disable specific devices in that list if you can. Maybe one of them is interfering. I will also update my submitted issue on insider.halowaypoint.com in hopes of them fixing this in the future.
EDIT: link to a screenshot of the settings page I mean: https://i.imgur.com/J9l5rW1.png (not allowed links yet on this forums so just copy & paste it for now)