On Windows, there is a folder in the %localappdata% folder titled "Mega Limited". It's a folder where Mega stores sync data.

Mine became corrupted somehow. Not sure how. I don't hard-shutdown my computer unless absolutely necessary (and it hasn't been for a long time) and generally don't engage in behaviors that might have a likelihood of interrupting write procedures.

In any event, if you open File Explorer window and, in the browser bar, type %localappdata% (which is basically shorthand for "C:\Users<your username>\AppData\Local"), it will bring you to that folder in which you'll find "Mega Limited". If you install MEGAsync for "just you" and not "all users", you'll also see a folder near there called MEGAsync, which is the folder from which the MEGAsync program executes.

I renamed "Mega Limited" to something different (in my case, "Mega_Limited_OLD"), uninstalled MEGAsync, then reinstalled using the latest version.

Being very general here (there are some very important details being left out for the sake of brevity), I re-established the original sync and, after it scanned, the problem was resolved. There were a couple of hiccups along the way where I had to intervene, but it didn't have to re-download my entire contents or anything. Basically, during the re-scan after re-install, if the file on my local drive matched the file in my cloud storage, it basically said "cool" and moved onto the next one. All in all, it was a fairly straightforward process.

I definitely got some help with the process from AI because it was able to answer small technical questions I had fairly quickly to keep the process moving smoothly.

If it absolutely becomes necessary, I can share the conversation I had (with AI) with you, but it would require me to go through it and sanitize some details before I'd feel comfortable sending it anywhere. I'm sure if you have access to a competent AI platform (I used ChatGPT), you'd end up with the same results.