Sorry to hear that you are having such troubles. A few steps that helped other users which I hope could be of assistance to you too :
1. For the Steam Version : Deactivate the cloud both on the game (Proprieties -> General -> Steam Cloud) and on the application (Steam -> Parameters -> Cloud -> Uncheck "activate Steam Cloud synchronization with compatible applications")
2. Remove the entire Solasta Folder from C:\Users\...\AppData\LocalLow\Tactical Adventures\Solasta (make a copy of it elsewhere to not lose all your data)
3. Start it to see if it works (From Steam preferably)
4a. If it worked, one of your files might be corrupted due to the double installation (MS Store + Steam). If you have any characters or saves you wish to add back, I would advise to only add back the "Characters" folder and "Saves" folder one by one, and checking in between that everything still works (to see if one of your character or save is corrupted).
4b. If it is still not working, I would advise to uninstall on both MS Store and Steam and only re-install the game on one platform (Steam without the cloud activated preferably) to at least check if it can be run this way without issues.
Other solutions that worked for some users :
1- Update the graphic card drivers
2- Add an exception to their anti-virus (especially if using Avast or Norton).
If none of that works, could you please send us your Player Log at firstname.lastname@example.org ?
You can find this file in C:\Users\...\AppData\LocalLow\Tactical Adventures\Solasta
(To be safe, you can send us your "Logs" folder if there is anything as well as the Player.txt and Player-prev.txt)
Thank you for your time!