Seemingly totally random 500 errors

Hi there!

I have a problem I've spent hours and hours on, but I can't seem to fix it, nor can my hostings support. So desperate as I am, i'm mailing you guys...

The website https://flowing-cosmetics very randomly shows 500 errors. I tried everything to fix it: make a new clean htaccess file, copy over files from the original wordpress zip, turn off all plugins, increase memory for wordpress, etc etc. I even moved hosts. Nothing helped.... I

If I change something (turn off a plugin - whatever really), the problem page usually loads again and then another page has the problem.

Sometimes I think it is fixed, but then it's back again. Just try some pages, and you'll probably get a 500 error suddenly somewhere. It's totally random.

Because I tried just about everything, I'm thinking it's maybe the theme (Studiofolio). But the developer does not have other issues, he says.

I really hope someone can help ...

Thanks! - Floris