The admin has shared a file folder with several team memebers and they are not able to access the files because they keep getting the message "Error Cannot open Excel because the file format or extension is not valid". While I am able to open the files just fine, and if they download onto their desktop they are able to open ok too.
Just to clarify, you mentioned that after downloading the file locally, it can be opened without issue. Are you receiving the error when trying to open files through the website with the Microsoft Online integration? Would you mind sending me a screenshot of that error?
Also, can you let me know what file extension the files currently have (eg. .xlsx)? Do you only have this issue with Excel files specifically?