It's unlikely something on the VFD has changed to increase the loading of the signal. However before you spend £50 for a new BOB it would be good to know for sure it's faulty.
You could substitute resistors for the VFD. I would guess the VFD input is equivalent to about a 10K resistor and would expect the BOB to handle a lower value of 5K .
Also you could use your meter to measure the current draw. Improper connection of a meter set to measure current can damage the meter so I don't recommend it unless you have enough experience and understanding to do it safely or know someone who does.
Troubleshooting the board without a detailed schematic is difficult. It maybe a faulty voltage regulator, a poor solder joint, a bad electrolytic capacitor or many other things. Again it would take someone with experience to do this. You could beg cnc4you for the schematic but I doubt they would be willing to share it.
If you do buy a new board the old one could be used to add IO for more limit switches and outputs should you want them.