@QkiZMR please go through http://stackoverflow.com/questions/4731364/internal-error-500-apache-but-nothing-in-the-logs
In this link they have shown a way to view the errors.

@QkiZMR Did you try setting error reporting to development in the joomla configuration? Also enable joomla debugging please. On which view (url) does the error show up?

@kshitijSharma2014 thanks for that link but this not help. But after changing php to verbose mode now I got blank page. In apache access log is:

...GET /blog/links.html HTTP/1.1" 200 239...

After changing error reporting to development joomla admin panel stops working. In browser I see "ERR_CONTENT_DECODING_FAILED" (Chrome). But /links.html page show errors:

Fatal error: Call to a member function setState() on a non-object in /var/www/qkiz.pl/blog/modules/mod_related_items/helper.php on line 44

sorry, I clicked 'close issue' accidentally

Ok, after disabling Related Articles module everything back to normal. Which means that is not Weblinks problem.
Problem appears on not articles pages like weblinks or contact form. Is Related Articles module is official Joomla module?

That was fixed with joomla/joomla-cms#9519

Is that fix is already in 3.5.0?

It'll be in 3.5.1 as it was patched after 3.5.0 was released.

I patched helper.php by myself with patch from link. Thanks, problem solved.