Grey leaves

Good morning everyone,
many of my African violets have started turning grey and dying on the new central leaves . They’ve been sitting in front of a bright window for several years, watered from below and occasionally from above with a spout, being careful not to wet the leaves. This is the first time I’ve ever had this problem.

I switched fertilizers a while ago, moving to Miracle-Gro, but I don’t think I’ve overdone the dosage. There was a period this summer when they might have been overwatered, although that was quite some time ago.

What do you think could be the cause — fungal disease, virus, root rot, or maybe burned roots?
Also, the only plant that’s flowering right now — which should produce double blooms — has made flowers without any petals