yEd 3.10 and yEd 3.11 do not support transparent fill colors for BPMN events. Either use only fully opaque colors or stick to yEd 3.14.
To correct your file, open it in yEd 3.14 and change "Fill Color" and "Fill Color2" in section BPMN Event to opaque colors. (Or use the corrected version. )
That said, the error message you posted has nothing to do with color opacity, but is the result of you trying to open a file that no longer exists. (I guess you used "Recent Documents" even though you already moved the file to another location.)