What could cause this?
Can you move the plugs within the screwing? If yes, than that is surely causing your problem. You can then warm the connector with a hairdryer or some other device and try to thighten the screwing. Yes...it is not easy. Mine was hard to screw, too.
Yes, that's a method worth trying, but otherwise, you can try another SD card.
Also please upgrade the latest firmware.
There is no park folder. It seems like it is a cabling problem. It is hard to tighten the contact.
Did you check the park folder? Can the files in Park be played?
If you are still using the old firmware, you can try to update the new firmware.
Are all files of the same size? Or are the corrupted files smaller than the working one? Videos gets corrupted when the K2 is not able to close the files properly. For example if the K2 crashes or looses power.
First thing you should check is the power connector. Push and pull both parts and check if it can move. If yes, it is not tightend enough. Some power connectors are a little bit malformed and therefore hard to thighten, but you can warm the up a little bit (e.g. hairdryer) to make them fit.
What firmware is currently installed on your K2?
Are all extensions of the files the same?
With which program do you try to open the files?
Do you see strange file names?
In short, the more information, the more chance we can help you. ;-)