Filament run out detector not working

A few things, make sure you are using the hole that is toward the outside edge of the printer, there is only one sensor. You can disable the sensor in OctoPrint settings via the web interface. Once disabled you can make sure that the switch used to sense filament is working properly by taking that black box off the back and inspecting it. If it isn't fixable by you then call Robo support to get additional assistance.

 

Yep, two holes only one of which works (when it works)

 

Well, @WheresWaldo might have a better idea on the R2.
On the C2 the two screws that hold that box to the case can be reached from inside the top of the printer (and on the C2 that means removing the top black "lip" on the case to be able to reach down in there, the R2 may have more room)

 

In order to remove the little black box containing the runout sensor:

1. Disconnect the wiring harness from the extruder
2. Remove the eight screws from the top section of the printer, there are two on each of the four sides inside the top behind the rails.
3. Lift the entire gantry off the top of the printer.
4. Behind the now exposed area covered previously by the Y stepper remove two screws that go through the back of the printer.
5. Reverse the procedure to replace everything.

 

That is a much better approach than what I used on the C2 and while I did less disassembly I did more cursing when screws were dropped