hdmi_safe will set the resolution to 640x480, if your monitor supports a higher resolution, you can set it by using hdmi_group=2 and hdmi_mode set to one of the values detailed in https://www.raspberrypi.org/documentation/configuration/config-txt/video.md. You'll probably have to add hdmi_ignore_edid=0xa5000080, my guess is that your monitor supplies a wrong EDID and the Pi tries to set the video resolution to an unsupported value.
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