We've not had any other feedback about a similar problem and I've not read anything about it on forums either.
There aren't many system settings that could affect this. In Settings - Device - Display - Screen Resolution, set it to something other than auto. In Settings - Device - Display - Screen Position, set it to 100% (if this causes overscan on your TV change your TV settings to a mode which scales input signal to TV).
I don't think the above will fix it though. It sounds more likely that it's a TV problem to be honest. On your TV what settings are there for how HDMI input signals are displayed? If there are different modes (i.e. PC mode, stretch to screen etc) try them out.
It could, I guess, also be a very unusual hardware problem with the MiniMX. If all else fails email us to arrange RMA. mail@futeko.com