Hi anonymous user-7489,
Please check whether you have broken lookup column in the default document library.
Make sure all lookup columns are hooking on existing lists.
Note: please remember to refresh SharePoint Designer if you make any changes on the document library.
Similar issue for your reference:
https://drewmadelung.com/sharepoint-designer-error-could-not-save-the-list-changes-to-the-server/
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