See below from Autodesk‘s release notes:
The Autodesk Civil 3D 2021.0.2 Hot Fix addresses a stability issue that occured when opening a drawing that contained either unloaded or circular xrefs. To resolve this issue, install the Autodesk Civil 3D 2021.0.2 Hot Fix and Autodesk Desktop Connector version 126.96.36.1993. For more information, see the Desktop Connector Release Notes.
This hot fix also contains the Autodesk Civil 3D 2021.0.1 Hot Fix which addresses an issue that caused Civil 3D 2021 to close unexpectedly when opening a drawing with a pressure network pipe run that contained curves.
When opening certain drawings from BIM 360 using Desktop Connector, the following error message may be displayed: “The process cannot access the file because it is used by another process.” This error may occur if a drawing contains circular or duplicated references, and may result in broken links to the referenced drawings. If you see this error message when opening a drawing, resolve any circular or duplicated references in the drawing when possible and save the drawing. Note: Drawings that are stored in BIM 360 may be opened as read-only if they contain circular references. To resolve circular references, you can copy the drawings that contain the circular references to a local folder by dragging and dropping them in Windows Explorer, open the drawings from the local folder and use the XREF command to resolve the circular references, save and close the drawings, and copy the drawings back to their original folder (overwriting the original drawings).