I have been using this one for years, it used to drive two 4k displays at 60 Hz. But since half a year or so, I can't get the second display to run at 60 Hz any

I have been using this one for years, it used to drive two 4k displays at 60 Hz. But since half a year or so, I can't get the second display to run at 60 Hz any more. I think it's a regression in the Linux kernel, but it's basically impossible to debug. A few times it suddenly did recognize it can do two displays at 60 Hz, but it seems to depend on the phase of the moon :/