On Retrospect 15 for Mac, if you are doing a backup of a SMB/AFP share and encounter the error "Scanning incomplete, error -1,011 (API request impossible)", please follow these steps to resolve the issue:
Open "System Preferences" > "Retrospect" and stop the Retrospect engine.
/Library/Application Support/Retrospect/retro.ini with a text editor.
Locate the "[Options]" entry and add the following line directly underneath of it:
Open "System Preferences" > "Retrospect" and start the Retrospect engine.
Restart your backup or copy. The issue should be resolved.
Last Update: March 21, 2018