I  came across one reason for the Error 500 problem, which was quite unknown to me. I publish it here to make others wise to the situation.

The other day I got Error 500, but the server was not actually down (for I could work on my Cpanel File manager), and it was not to due any change in plug-in settings. My site is hosted with TMDhosting (shared hosting). I asked them to help me out.

George Moody of TMD not only resolved the problem, but also explained the reason: “Actually, it was caused by the fact that your application is using the global /tmp folder to store some temporary files. The tmp folder has a limit of maximum files that can be stored simultaneously and it is usually used for sessions only. As the maximum number was reached the application reported an issue. I have flushed the tmp folder and your website is fully accessible now.”

I asked Moody: “Is there any arrangement to flush the tmp folder regularly, and safely?”

Moody responded: “As this is a global server folder I have already applied this in case it affects others as well. It will be safely flushed on regular bases. I am sorry for the inconvenience caused.”

Maybe you can check with your hosting company to get it confirmed whether your site is taken care of in this respect.