This is the first time I get this error.
This ocurred after creating a new custom module, which comes with joomla by default.
After that, I got blank page and the following error:
Fatal error: Cannot redeclare class JEditor in \\WAGNER\WWWROOTAMENPT$\xxxxxx.hostnd\htdocs\www\libraries\cms\editor\editor.php on line 554
It seems that it happens after trying to create this custom module, but I already deleted and it's still showing up.
I haven't installed any new extention and everything was working fine, until I created this custom module.