Okay, sections/galleries work again, thanks for fixing that, Graham.
But there is another problem, and I can't even figure out what causes it...
Look:
http://realities.simpworks.com/ - works just fine;
http://realities.simpworks.com/webtips - shows an empty page and returns "500 Internal Server Error" code.
http://realities.simpworks.com/category/art - works just fine;
http://realities.simpworks.com/gallery/img - shows an empty page + 500 error code.
Some pages work, some don't... the weird thing is that if you add any random parameter to the address, even a single question mark:
http://realities.simpworks.com/webtips? - works fine now

I never met such symptoms before, all pages should work. My only suspicion is a badly configured internal proxy on the server; looks like it has cached those "bad" requests and now returns "500 Internal Server Error" for all of them, without launching Apache and PHPC, ignoring even incoming "no-cache" headers.
Any help?