Go into web.config and change customErrors to Off, then try again and email the error message to support@dotnetinvoice.com.
When we've worked on your installation in the past there were problems with the SQL database setup (if I recall, the initial setup script ran using a tool that truncated the script so a bunch of objects are missing from the DB). It's very likely this is a problem with a missing stored procedure. It will be easy to tell from the error message you receive. If you run into problems getting to a real error message please email us at support@dotnetinvoice.com.
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Rob
DotNetInvoice Support