Based on the message, it does seem like a connection string issue but I wouldn't think it was because the assemblies do not have a strong name (I run the debug, unsigned assemblies all the time).
You could try to find any differences between the two deployments (working vs. non-working). Since it looks like it's a connection string problem, and this is just the dev server can you output the connection string? If that looks OK then you
could try some simple ADO.NET to open a connection and see if that is the problem.
Another strategy is to try and break the application on purpose: give an invalid connection string or remove the connection string. Sometimes this gives an error that you wouldn't expect and can lead you to find the issues (the virtual root is point
to the wrong place, another web.config is interfering with my settings, assemblise are not reloaded, etc.).
Enterprise Library support engineer