I don't think this is a problem you will get to the bottom of, unfortunately. I've seen this issue every so often on every version of Excel I have used over the past decade or so. Ctrl-Break twice plus F5 fixes it for me when I come across it. Before I learned that trick, resetting the computer would also generally fix it.
I second skywriters comments on selecting, activating, etc... I would also add that when working with more than one workbook or worksheet, it is very important to clearly identify which workbook or worksheet a range is part of. This might help get you started:
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