Here's the deal. The right hand shifter (rear derailleur) on my winter bike plays tricks on me when it's cold outside (below 5 degrees celcius). It won't shift down the cassette. It's like the paddle won't properly "grip". When the bike is in my flat (warm), it's no problem.

I'm thinking it might have to do with some old grease inside the shifter. Something inside the shifter that thickens when it's cold. I have been looking for information on how to service (or just take apart) the shifter, but so far I haven't found anything.

Does anyone have an idea?

I offer no guarantees, and I've never worked on that specific shifter, but in some cases you can blast a shifter out with GT85 to unstick it. You may then want to add a little fresh grease so it's not too dry, but that's a matter of judgement.

Flush with a lube/solvent solution. I use CRC Powerlube. I had the exact same shifter and similar symptoms. Flushed with Powerlube and the shifters worked perfectly for years, before I passed them on to someone else.

Don't take it apart. Just remove the hoods, flush every cranny of the internals liberally with Powerlube (while working the shifter) until it drips out relatively clean. You could also try swinging the shifters to work the solution into the internals.

Keep working the shifting and you should see an instant improvement. Clean off and you should be good to go.

Thanks a lot for the suggestions, it's greatly appreciated