Index File: An Error Occurred while trying to complete your request

Update! I solved it. I had to go into my steam save game settings, find where it was saved and it was in documents but NOT user specific. Through going into my hard drive through file explorer and clicking on the folder location I found out I didn't have access despite also being an admit. Simply clicking "continue" granted me permanent access and got the game working again.

Update: I do not have this issue when using a windows account on the same computer. Both have admin permissions. Why??

Just had this myself, but having found another thread after seeing this one, a suggestion there was to check that Windows Defender hasn't suddenly decided to block the game's access to the files on your device. Seems to have worked for me 👍🏻

Edit: apologies, having just re-read your post I can see you did already try this yourself, my bad. Can't offer any other tips myself, since this did work for me so you're clearly facing a different issue, sorry! Hope someone is able to help you out soon! 🤞🏻