Hi there, I'm new to forums.

So i bought this locking "Planet Lock" strap. It seems very robust and secure.
However, since I started using it, my screw for straps screwed off (not completely) two times on my old Washburn G 5V. My thoughts are that the strap being not very flexible and a bit hard, the weight of the guitar makes the strap "pluck" the screw off by force?

Is this a guitar issue or is this strap plucking the screw out and damaging the body of my guitar? Should I get a leather strap with a strap lock instead?

You could always put a longer screw into the guitar. It may just be that the existing screw isn't long enough. If the guitar is old, the screw could be working its way loose anyway. Or do the old toothpick trick. Break off a toothpick in the screw hole and put the screw back in. The toothpick will give the screw something to bite and it will take up any slack in the hole.
I started using Planet Wave's oblong strap buttons lately. I have them on almost all of my guitars. The only one that has straplocks right now is my LTD explorer because it seems to hold it at the proper angle better with them on. With the oblong buttons I don't need locks on anything else, and I can use any strap I want to.
I don't know why they even make circle strap buttons anymore.
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I've used them extensively without incident. It's likely that you've got them cinched down too tight, and then whenever your strap twists you untwist it without thought of direction and it unscrews your strap button screws.

FWIW, I crank mine as tight as they will go as well, I just pay attention when I straighten my strap.
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Sounds like your buttons are loose. I have a few of these that I picked up for next to nothing and they work great.
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I've used one of those straps for years without incident. Although some guitars it doesn't fit right. Just make sure the screw is tight when you first pick up your guitar. One of my older guitars had a loose screw but i got into a habit of checking it when i picked it up. Now, I always check the screws out of habit, no matter the guitar. Only takes bout 2 seconds. Or you can fix it pretty easily as mentioned above.
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