Welcome to the world of Micro-Crap.
Since CP/M (the ancestor of DOS and therefore Windows) was never meant for multitasking, it did not have file-locking implemented. So a lot of applications worked around that defect by writing the file-locks to the files themselves (a horrible concept, but that's what you get when a law-school dropout tries his hand at programming), and that includes MS Office.
Those products are very clever random number generators in terms of function. Now they work, now they don't.
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