This issue is mainly caused within Microsoft office word as the document is being co-authored. This is a common error in co-authoring. This happens when your document cannot be saved to the server because your edits conflict with edits from others.

When you receive upload failed-conflict error and Your document can't be saved to the server because your edits can’t be merged with changes from others. Please try the steps given below to resolve it.

Note- The error message will be encountered only in few documents and not all report documents will be affected.

• When you receive this error, click on resolve

• Click on drop down of accept conflict changes and select accept all conflict changes or accept/reject the changes you would like. Save the document

Article for your reference: https://support.microsoft.com/en-us/office/troubleshoot-common-coauthoring-errors-f0a0be0c-c03f-45b2-bec0-535e5b13f691

and

https://help.workplace.co/hc/en-us/articles/115000388333-Resolving-File-Conflicts-for-Office-Documents