Dendrobium leaves are turning yellow and fall off

[Alya091]

Hello guys,

So, i bought this dendrobium on 29 August and repotted it the next day with Seramis Spezial-Substrat für Orchideen Mix and add a light layer of spaghnum moos on the top layer. Until now i haven't gave it fertilizer except twice weekly sprays with Growth Technology Orchid Myst Spray (as with my other orchids). I put the Orchid inside my room on a east-facing windowsill. Lately (since around two weeks ago) the leaves started turning yellow and fall off and continues to do so until now and now some of the canes/pseudobulb doesn't have any leaves left. Did i do anything wrong? is it the wrong Substrate? (cause my 3 other phalaenopsis orchids didn't seem to have any problem with the Substrate) or is it a cold damage? The weather here in kassel rains quit alot or could get cloudy/misty the whole day. I'm pretty sure this is not a dendrobium nobile either, so it couldn't be a winter resting period, could it?

Any thoughts and advices would be greatly appreciated!
Alya