I use VWD 2005 and do get errors reported that are bogus errors that are handled when the page is rendered. For Instance:
Error 3 Validation (XHTML 1.0 Transitional): Element 'html<%= HtmlAttributeList %>' is not supported. D:\My Pictures\website ideas\DNN5.1.0\Default.aspx 4 2 D:\...\DNN5.1.0\
But, I also get a lot of warnings about things like this:
Warning 26 'Public ReadOnly Property HostSettings() As System.Collections.Hashtable' is obsolete: 'Replaced in DotNetNuke 5.0 by Host.GetHostSettingDictionary'. D:\My Pictures\website ideas\DNN5.1.0\portalContentsImages\5\Default.aspx.vb 326
And this installation is a fresh install of the full package for 5.1. It seems odd that the default.aspx.vb is outdated with deprecated stuff when there were no upgrades from one version to the next, this was a new install of 5.1 on localhost. It really shouldn't occur in a clean full install. I use the automatic install btw, just easier and it always works for a localhost install.
However, my site compiles and runs fine in spite of the bogus errors and the odd warnings... I don't see any errors about module actions, etc. I guess I figured since it compiles and runs fine, it's probably just VWD not understanding DNN and how the errors are fixed when the page is rendered.
Perhaps try parsing the skin if it is an html based skin so the ascx can be generated again in DNN 5.1 with any appropriate release related changes. I got a bunch of errors about 'skinpath' being changed to 'containerpath' when I loaded a skin created in DNN 4x and didn't reparse it (I did an ftp skin install instead of the skin wizard install). The other thing to try is to use the standard actions menu instead of the drop down actions and see if the problem is just with the drop down actions code.