This error message applies to Ultimaker 3 an Ultimaker 3 Extended printers.
A rare error can occur on your printer reading Line_NR with a string of numbers after it. This error message indicates that the gcode your printer is trying to read was corrupted, and the printer experienced problems reading at that specific line.
This error is the result of an unsafe ejection of your USB stick from the computer. It doesn't have to have been the last time you removed the USB stick from the computer; it can have happened multiple removals previously.
If you receive this error, you will need to reformat your USB stick (it should be in FAT32 format). This should resolve any corruption issues.
Always use safe eject procedures for your USB stick when removing in from the computer in order to prevent this.