I was getting this error for quite sometime.
“soap:ServerServer was unable to process request. ---> Cannot complete this action. Please try again. ---> Cannot complete this action. Please try again."
Finally found a solution below:
Go to Site Settings > Master Page and Page Layout Gallery, checkout the file through the browser. Once checked out to you click on the dropdown menu of the file again and click on "Edit in Microsoft Office SharePoint Designer" and voila! It should work!!
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