If you are encountering this error, then the printer has detected an invalid project file to be printed. This error likely has occurred because the file is not properly sliced for the Ultimaker 2+ Connect.
Please try re-slicing the print job in Ultimaker Cura.
Additionally, when saving the print file to USB, make sure to safely eject the drive before removing it from your computer. This prevents possible problems with corrupting data on the USB device.