Weird referencing issue

[Giordano Bruno]

This has just started to happen. I open a new workbook and type 2 in cell A1. I add another worksheet and in this worksheet I select cell B4 and type "=". I then switch to the first worksheet and there is no indication of the active cell. I click on cell A1 and the formula bar says "=Sheet1!A1" this formula appears also on top of cell B4 in Sheet1, although there is nothing there. I then press Enter and I am returned to Sheet 2. My active cell is still B4, but the formula +Sheet1!A1" and the result (2) appears in cell A1. The destination cell B4 is blank. The problem occurs every time I try to "point" to a cell or group of cells on another worksheet. I've tried the Excel Quick Repair and the Repair for the Internet and in desperation, I uninstalled Office 365 and reinstalled it. If anyone has any suggestions, I'd be very grateful for their help