You must SSH into your box (there are lots of tutorials about doing that if you are unfamiliar with it).
When you arrive at the command line after SSH’ing to your box, just type this:-
fw_printenv hdmi_mode and enter. Then repeat for fw_printenv output_mode.
These commands show what the box setting is for these values. If your problem was the same as mine, you should receive “mode not defined”.
To change the values type:-
fw_setenv hdmi_mode 1080p60Hz then enter
and
fw_setenv output_mode 1080p60Hz and enter again.
Reboot your box and that should fix it.