Help! Brassia leaves turning yellow

[Nausicaa_*]

Hello everyone,

I have a problem with my Brassia, I don't understand what's going on.
Usually, she receives no direct light but in this period is quite dark during the day in the Netherlands.

Temperature at home

Tday: 18-20 °C

Tnight: 16-18 ° C

Repotted on the 16th of June because roots were too wet.
I used specific fertilizer.
She was a happy and healthy plant. My sympodial orchid forms a new pseudobulb.
But now..something is happening.

Could be something related to the amount of light?
I have read the opposite, leaves turning yellow when they received too much light.
Or a disease?

I don't have any clue.

I hope someone can help me.

Thank you in advance!