Dendrobium Cuthbertsonii with purple spots on leaves

[slovakinaustria]

Hi!

I would like to kindly ask you for an advice.

I grow Dendrobium cuthbertsonii in a terracotta pot in a mixture of sphagnum moss and fine bark. Daytime temperature is between 22-24 C (72-76F), nighttime temperature is between 10-14 C (50-57F). Humidity during day is around 70-75%, during the night slightly above 80%. It receives 13 hours of bright indirect light a day and fans are running 24h a day. I water it with distilled water every 2-3 days.

The plant was doing fine for some weeks, But recently it developed dark purple spots on its leaves over night and even dropped some leaves the following day - it is possible the temperature during that night dropped a bit lower than usual, possibly around 7C (45F) and the purple spots appeared on more leaves following days.

Is this a cold damage or is it a bacterial disease? Or is it anything else? Any help? I am new to growing this specie...
Please, see the picture for refence.