microSDcard

How are you moving the videos?

First thing: back up the content of that card to something else (a computer for example). If the card is dodgy nothing on it is safe. And don't move or copy anything else to it until you have done so.

Can you copy the bad files back and do they work when on your phone? If so then the data are good and the problem is with your video player or file manager.

Otherwise it depends on what the real cause of the problem is. The card could be failing, in which case frankly a new card is the best answer. It could be something with the phone's card reader/writer - maybe rebooting the phone or reseating the card might do it, or maybe it's a hardware failure. Or it could be that the card is a fake and you've hit its real capacity and now it's overwriting data. Although it hardly seems worth hacking a 32GB card to pretend to be 64GB, the fact that you say the problem started when you reached 32GB is potentially suspicious (though that might indicate a software problem rather than the card, though I've not heard of one that produces this behaviour).

So I'd back the card up completely then do some tests. Does a different card work OK in the phone? If you remove stuff from the card does it start working again? Is it only videos affected? That might suggest that it's not the card itself that's the problem. And if you've any suspicion that the card may be a fake (i.e. programmed to tell the device that it is something it isn't) stick it in a card reader, attach it to a computer and download a utility called h2testw and check it out properly (after backing up all data).

And if you have any doubts about a card at all, or regard the data as important, always copy rather than move. You can erase the original later if the copy is good, but if you move and the copy is bad then it's gone.

I never move items always copy, this way you have something to fall back on

THEN you can delete the files once they have transferred...
The "move" option is too unpredictable in my experience when dealing with big files