You can also try and wipe the contacts of the memory card, i have some times experienced a card would not work after copying footage off the card, but then a quick card wipe on a sleeve fixed that.
Also testing another brand camera i had issues with it booting in the "morning" in just under freezing temperatures, but then a few days later it was 2 X colder and now it worked fine, and a little later again just below freezing and then the damn thing worked just fine..
And it have been so ever since, though now it also seem like winter are over for Denmark, not even freezing temperatures at night these days.
Regarding rear camera issue, it is something easy to test just pull its plug out of the front camera and do a few tests / drives / days to see if the front camera alone are fine, it is most often a wire issue, happen much more often than i have heard of faulty rear cameras.
You could also run a test on the memory card on a computer with the card in a card reader, i have lately experienced a brand new card that was supposed to have a 80 MB/s write speed only be like 20 MB/s, doing a full format with sdfoamatter restored the cards speeds, though it was still not working in any dash cameras, only now months later have a found a dash camera that card work in aside for my DJI Osmo action camera.
Cards dying fast i have also experienced a few times, even before the Damn thing filled up 1 time, and this was back when 64 GB was a huge card,,,,, and expensive. i got both of those cards replaced just fine.
You could of course also have a defective camera, but trying all the obvious things will it make it more easy for Viofo to do a RMA for you, and if it is something simple you do not have to go thru the hassle of doing RMA / take system out of car.
SDformatter you can DL from the SD associations web page.
www.sdcard.org
Another free little tool to test are H2testw, this little tool will fill your card with test files as it test read & write speeds
Formatting ( full ) or running H2testw will of course take its time, not least with a big memory card, fortunately you can do other stuff on the PC meanwhile, or do something more productive.
O and just to be on the safe side.
How are you powering the camera ? if you use anything out of the provided stuff, that might be a issue,,,,,, i have lost count how many times in the past 10 years people have come here with problems, and only after 3-4 pages of this, we found out the camera was plugged into some USB outlet in the car, and then changing to the provided source of power cured the problem.
