We do reviews via shared commenting on our SharePoint site. Reviewers using Adobe Acrobat Reader DC 2021.005.20060 on Windows 10 are experiencing the following error when they click Check for new comments:
First a SharePoint dialog appears that never appeared before. If this dialog is closed the message " Could not get new comments. Login credentials are expired or invalid; please re-enter your sign-in for access." appears.

Expected Results: Any new comments should appear, or a message "no new comments" should display.

This issue is also noted here (seems to impact other Acrobat versions too): https://community.adobe.com/t5/acrobat-discussions/could-not-get-new-comments-login-credentials-are-expired-or-invalid-please-re-enter-your-sign-in-f/m-p/12370427