Need advice for my Phal with rotten roots, pls.

[awach]

Hello, I'm a new member, so hello again. So, I have this phal flowering beautifully since November last year and still is in bloom with two spikes full of flowers. However, she had very nice healthy roots when I got her, but I have decided to repot it because the medium was breaking down. So I did put her into mix of fresh spag moss and orchid bark. And I have noticed today roots are getting mushy and rotten. So, I have only few good roots left, however the whole plant is very good condition. Her leaves are very firm and stiff, very green, there is also a new leaf coming out. Flowers are very nice looking, but now I'm afraid I will loose this phal, as I have some already. So I really do not know when and how I made a mistake of overwatering. Any suggestion for rescuing her when she is still in very good condition.