Thanks for pointing this out. Yes the missing ltdl.dll from GraphViz distro is the problem unfortunately preventing WhiteStarUML from running unless you have full GraphViz installation. That dll was added to version 5.2.3 being uploaded right now, additionally GraphViz dll is being loaded dynamically at runtime now so if it backfires again it should not make more damage than preventing diagram layout from running.

The other problem is a consequence of WhiteStarUML not being in an executable state when the installer tries to register it as a COM server. To fix it you may register it manually with the command:
whitestaruml /regserver (as admin)
After that all errors msgs about invalid class should disappear.