Fatal error: Cannot redeclare class WPAlchemy_MetaBox in /www/htdocs/w00c0eb9/wp-content/themes/

Fatal error: Cannot redeclare class WPAlchemy_MetaBox in /www/htdocs/w00c0eb9/wp-content/themes/nevada/lambda/lambda.meta.box.php on line 56

This is what I get, when I activate the Nevada Theme. It first just occured, when trying to preview an woocomerce product. Because of this, I deactivated the plugins and activated them again! Now I get this. What might I have done wrong, what should I do now?

Thank you for every help you can give!

Daniel

P.S.:

If necessary, you could even log in to the ftp !?