"File exists but couldn't be open for reading" / "Unable to open file" pop-up window loop

This is an issue I've begun experiencing this year.  I'm praying someone can help me.

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SUMMARY:

I will create and work on simple projects that use a combo of JPGs, mp4s or MOV assets.  AE will operate fine.  Renders will be fine.  Everything will be fine.

..But without fail, days/weeks/months later, I will go to open one of these projects and be met with an infinite loop of pop-up messages  Each pop-up will appear one after another, and they will say the same thing for each asset in my library

 1) "AE error: File Exists, but couldn't be opened."

and then

2)"AE Warning: Unable to open ____. If the file is open in another application, closing it may solve the problem."

...I will spend minutes clicking OK for each of these pop-ups, but they never stop.  Eventually I will always have to force-quit after effects.

I know it could be a number of things, but I'm most confused by the fact that things will WORK for a while, and then randomly stop working without any change having been made (No version updates no files moved, etc).

ANY advice is greatly appreciated.  Thanks in advance!