In case of local files the "filename" variable holds the string that is later assigned to the filename property (no difference which is used), but when playing URIs only filename property holds the string. Passing an undefined value to the label caused an error in this case and prevented playback of URIs.
The "clapper_src" directory name was unusual. This was done to make it work as a widget for other apps. Now that this functionality got removed it can be named simply "src" as recommended by guidelines.