cant change checkout

everytime I try to change my checkout options (I want to make it so no registrations is required) I get this error:

Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, This email address is being protected from spambots. You need JavaScript enabled to view it. and inform them of the time the error occurred, and anything you might have done that may have caused the error.
More information about this error may be available in the server error log.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

actually if I change anything on that particular page i get that error. But it only happens on that page. I can change almost anything else.
I think something is wrong with the server settings, but I don't know what. Please advise.

I am re-building the site because our last web hoster lost all the data. So I have changed web hosting companies.

Hi,

If you change the settings of, for example, a plugin and you save you don't have this error ?
Or when changing a setting in the Joomla parameters ?