LAST UPDATE + + +
What I did in the last 25 minutes:
// Deactivated the pulgin suggested
// I copied and pasted the SAME wordpress folder from my domain ONE to one folder into myDOMAIN2, using the same server (to save time instead upload everything again)
// I reviewed the 777 permissions ti the folder /contents
// I reconfigured the "wp-config.php adding a prefix to use the same database
// I went to the url "install" to install it
Succsess! it works now. The Error 500 has gone so I ran to the first installation and...
// I reconfigured the FIRST "wp-config.php used in my First installation (the same that gave me this Error 500) to use it in the same database adding a prefix. "Basicly I did the same as the before posted".
// I 'install it' and I login, I write , I test and it works now.
----- I don't know if this works for the errors of other users, because my Error 500 was raised when I tried Add comments or browsing pages. Just was my personal experience, this is not a Fix Key :-)