When things go bump in Sharepoint, typically you’ll see a helpful error message displayed, such as;

“An unexpected error occurred….”

Which is less than useless, you can switch this off so that the underlying .NET exception information is displayed instead; to do this;

Edit the web.config for the Sharepoint web application;

Change

<customErrors mode="On" />

to

<customErrors mode="Off" />

Change

<SharePoint>
    <SafeMode MaxControls="200" CallStack="false" DirectFileDependencies.......... />

to

<SharePoint>
    <SafeMode MaxControls="200" CallStack="true" DirectFileDependencies.......... />