Dendrobium phal. Leaves spots

[Nach_]

Hello, wanted to ask regarding my Dendrobium Phaleanopsis, as Im a bit worried something is going on...

I have had this plant for a while, bought it when blooming and everything was fine for 3 months (it lost its flowers but part of normal lifecycle). However, for the past month, I realized that some spots are appearing on the leaves. 2 leaves have already dropped (which could be normal, Ive been told), but the remaining leaves all look like the attached picture.

Root system look healthy and there is a new cane growing.

Any ideas?

Thanks a lot in advance!