"There was a problem completing this request" error when trying to share a link

I am trying to get a link to the file I uploaded. I used to be able to do that for about one hundred PDF files before. Since two months ago, I can no longer do that.

I have used 262.8 MB and I have 2.75 TB free space.

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I took one of a file that I have “Created” the link long time ago. Five minutes ago, when I right-click it, click “Dropbox” and then “Share,” it gave me the option to “Copy” the link.There was no problem to copy the link.

Then, I made a copy of that file and gave the copied file a different name. When I right-click it, click “Dropbox” and then “Share,” it gave me the error message of “There was a problem completing this request,” same as what happened to more than ten files I uploaded.

What’s the problem? Please help.