I will answer it in a very similar manner so that other users can easily make out in case they have a similar issue. So let me answer each of your questions:
Answer 1: the problem is with all excel files and sheets. I tried opening multiple, new and single files but it did not give any varying output.
Answer 2: the extension is .xlsx, I remember, my previous version of OS giving me .xls as the extension. How does this matter anyways.
Answer 3: I tries both. I have been trying both, infact but it did not give me any varied result either, it’s all one and the same
Answer 4: I don’t know to open excel in safe mode. If you could be kind enough to explain me how to go about it, I can definitely give it a try.