(In reply to comment #6)
> * with "echo 2 > /sys/module/drm/parameters/debug" nothing special in dmesg

Hmm, so maybe the EBUSY error is coming from this code in
drm_mode_page_flip_ioctl():

        if (crtc->fb == NULL) {
                /* The framebuffer is currently unbound, presumably
                 * due to a hotplug event, that userspace has not
                 * yet discovered.
                 */
                ret = -EBUSY;
                goto out;
        }

What does xrandr --current say when the problem occurs? After that, does
running xrandr without arguments make any difference?