Change of hosted domain..

Have you reviewed Changing_The_Site_URL?

yes, it’s done …

Do you still have a problem?

yepp…

Can you access your server’s error logs?

I’ve been trying to find it, but I’m not sure where exactly to look. Know
I have done it one time before

Which method did you use the change the urls?

I used MyPHPadmin

Try:
– switching to the Twenty Eleven theme by renaming your current theme’s folder in wp-content/themes and adding “-old” to the end of the folder name using FTP or whatever file management application your host provides.

– resetting the plugins folder by FTP or phpMyAdmin.

– re-uploading all files & folders – except the wp-content folder – from a fresh download of WordPress.

I don’t have twentyeleven in my themes-folder any more. Should I upload only that theme from a fresh wordpress?

I guess so

having trouble uploading seperate files… How do I add this theme again the easiest way?

Just upload the complete twentyeleven theme folder to wp-content/themes.

Done all that now, but still “500 error” on all sites

Did you rename your theme’s folder?

Check if the file permissions are correct.
This depends on the host’s setup but generally this will do:
All directories should be 755 or 750.
All files should be 644 or 640.
Exception: wp-config.php should be 600 to prevent other users on the server from reading it.

Because this is a 500 error ask your host if they have any problems on their end.