I saw this same problem after updating my Duet Web Control to v1.21. The error message is:
M32 "0:/gcodes/Spider.gcode"
GCode file ""0:/gcodes/Spider.gcode"" not found
Ffiled to open file "0:/gcodes/Spider.gcode"
In the second line it's adding an additional pair of quotation marks. It's not a long file name, and there are no special characters in the file name.
For the time being I have gone back to Web Control v1.14-b4, and everything is fine. Also running firmware version 1.17RC3 (2016-12-21) on a 0.85 board.