Alright, writing this now so I don't forget what exactly happened over the last few hours;
Got my elegoo mars 5.5 mono lcd the other day (my original lcd rapidly developed black dots in varying sizes so I figured why not upgrade), finally installed it and all was physically fine.
I went and updated the firmware (from ChituSystems site), had problems printing so I updated chitubox to latest version (x.9). Got 'invalid file format' when trying to print. Couldn't figure out what I was doing wrong whilst installing different firmware versions until I looked at the printer system information after updating to ChituSystems firmware.
The file version in the firmware file is (4.3.13_LCDC*)...chitubox is looking for version (4.4*) at a minimum.
So what did I do? I am using the elegoo 4.4 firmware while using the chitu 4.3 cbd and everything seems to work fine...so far.
T/S notes: I tried an earlier version of chitubox (*.8) and had all sorts of other printing problems (exposing the whole screen with weird distortions, that sort of thing). I also found that with the update to chitubox, you NEED to save to the 'ctb' format or you will get 'invalid file format'.
Maybe this helps someone out there.