Hi guys,
Let me share my experience with this exact same problem.
It started to occur last week for me (I have been using the Stocks feature for months now) in the file that we can call file A.
I first try to run Quick repair, repair Online and install again Office 365. No improvements.
Then I tried to see (which I should have started with) if the message would still show up in another file B containing the same stocks as in the original.
It did not.
I finally came up to notice that 2 specific stocks price were not updating (change% would still be 0 %) after each refresh in file A.
Then I noticed that one these stocks had a slight different spelling in file B compared to A and the other stock name had a different stock market reference (comes in parenthesis after the stock name in the same cell), this later one may actually come from my wrong doing(?).
So I copy/pasted names from file B to A and then the Refresh worked fine again, no Warning message anymore.
So I'm not sure exactly what happened, but I hope my experience can help others (and among them Microsoft teams so that they can come up with a patch or something).