Why is there a red LED flashing on my radiator valve?

When the TRV opens and closes and shows the error light, it is an indication that the range of movement is too short or too large. If the error light flashes with the On light showing also, then this an indication the range of movement is too large, if it is with the Off light showing, it is an indication that the range of movement is too short. Using the other adapters or ensuring the device is not over/under tightened will ensure the range of movement is correct.  

Few users have managed to resolve this and reported back the solution :  

I have been able to resolve this successfully, The pin in the TRV was not long enough to hit the pin on the valve It would try and calibrate, open and close the full way without successfully turning off the radiator by pressing on the Valve Pin.  

I have resolve this by inserting 2 x 1p coins into the valve "Channel" for want of a better description

So when the TRV Pin extends, it presses on the two copper coins that sit on top of the pin on the radiator valve.  

We have had users report success when they have "exercised" their valve seat itself which allowed our TRV a little more movement. Our TRV will push the pin as far as it can before its motor cannot push further. This will of course vary on each valve in the market, and we have adapters for the majority of TRVs. I'm not sure with your valves in particular, but we are aware that Danfoss appear to have some "non standard" seats and have produced their own adapters (for example http://www.heatingcontrolsonline.co.uk/danfoss-ravl-trv-adaptor-p-919.html) for use with other TRV heads in the market. This may or may not be related to your issue however.