I've been running an Apache server with 3 domain names setup through virtual host without any problem, until today when I tried to add a fourth domain. Now all my domains return a 502 error. I've removed the recent domain from virtual hosts, but everything still returns a 502 error.

At this point I'm pretty much desperate short formatting and reinstalling everything.

502 appears to be bad proxy. Can you try disabling whatever you're doing with mod_proxy and see if the server works without it? If it's fine then, you may have something wrong in your proxy config.

I haven't touched mod_proxy at all, so I really don't know what I could disable.