I fix this error by change the mod_mono.conf .

this is my mod_mono.conf

LoadModule mono_module modules/mod_mono.so
MonoAutoApplication disabled
AddHandler mono .rails .aspx .ascx .asax .ashx .config .cs .asmx .wgx .castle
MonoServerPath /usr/bin/mod-mono-server2

Alias /lib/ "/opt/mono/server/mono/site/"
MonoApplications "/lib:/opt/mono/server/mono/site"
<Directory "/opt/mono/server/mono/site/">
    Options MultiViews
    AllowOverride None
    Order allow,deny
    Allow from all
</Directory>
<Location /lib>
        SetHandler mono
</Location>

if miss the last three line,the page will report script error.

An advice to everyone who tries to get a VWG application running on Linux/Apache/Mono: Take the time to read some basic instructions about ASP.NET on Mono/mod_mono here. This certainly helps avoiding irritations and involving additional resources.

Bernhard