Thanks to those of you who have replied.  YOur suggestions have set me on the right course to hopefully coming up with a final solution.

After a lot of painstaking testing I've uncovered the following issues while attempting to update a DNN 5.2.1 site to 5.5.1 on my local server.

After deleting all unused modules and doing my best to determine that all modules in use were current (not all vendors replied), i ran the upgrade and received Success indication for all module BUT the site WOULD NOT OPEN.  I received the generic DotNetNuke Error Page and the very useful  (NOT) "An Error has Occurred" message.

BUT, I found that in IIS if I set ASP.NET version for the site to v4.0 instead of  2.0, the SITE OPENED.  This allowed me to go to the Event Viewer where I found that the Udassis Content Rotator Module (1.0.0) failed to load and was throwing an exception.  I am attempting to resolve that issue now.

QUESTION:  Is the IIS setting = ASP.NET 4.0 a requirement to run DNN 5.5.1.  If so, I don't see that documented anywhere.

QUESTION:  Anyone have an input or info of Udassis Rotator?  Is there a replacement or alternative.

Thanks again.