500 Internal Server Error – Full load memory and CPU

What plugins are you running?

Akismet, BWP Google XML Sitemaps, Flickr Manager, Wnagguard, FB like and google plus one plugins.

Okay, none of those would do it (BuddyPress is known for kiboshing things on small servers).

How many sites do you have on the network?

4200-4300 sites and 5-6000 visitors / day.

Oh that’d do it if your server isn’t optimized for WP.

Read https://thethemefoundry.com/blog/optimize-apache-wordpress/

And look into turning on some sort of caching.

Okay, I send this page to my webhost. I don’t have root access.

At this many sites, you need root access. Or at least a VPS.

I’ve cloud webhosting with 256MB RAM & 20% CPU load.

I’m sorry, but are you saying you run 4200-4300 sites of 256mb of ram?

And at 5-6000 visitors a day, that’s only ~1.5 views a site a day. There is something wrong with those stats.

I was assuming he means 5-6k per site, not total. If it’s just total, though….

Summary.

My webhost say:

“We can’t change these settings, because others also use this server…”

You’re probably on shared hosting, yeah. Not a good fit for a multisite that large.

Cloud hosting… ??

Cloud can be shared and if you can’t edit your settings, it’s the wrong hosting for you.