Sorry, that was not the kernel. It was (is?) a bad usb socket. I plugged the monitor to a different usb-c port and it works without complaints on all tried kernels (5.6, 5.8, 5.9).
Device: "20NYS8QG0B (LENOVO_MT_20NY_BU_Think_FM_ThinkPad T490s)"
Sorry, that was not the kernel. It was (is?) a bad usb socket. I plugged the monitor to a different usb-c port and it works without complaints on all tried kernels (5.6, 5.8, 5.9).
Device: "20NYS8QG0B (LENOVO_ MT_20NY_ BU_Think_ FM_ThinkPad T490s)"