Hi Sebastian

We have more or less some of the same issues. Are you able to figure out the issue based on  the following?

2018-03-26 16:40:52,185 [WINWEB03][Thread:19][ERROR] DotNetNuke.Framework.Reflection - Dnn.PersonaBar.Pages.MenuControllers.PagesMenuController, Dnn.PersonaBar.PagesSystem.IO.FileNotFoundException: Could not load file or assembly 'Dnn.PersonaBar.Pages' or one of its dependencies. The system cannot find the file specified.File name: 'Dnn.PersonaBar.Pages' at System.RuntimeTypeHandle.GetTypeByName(String name, Boolean throw , Boolean ignoreCase, Boolean reflectionOnly, StackCrawlMarkHandle stackMark, IntPtr pPrivHostBinder, Boolean loadTypeFromPartialName, ObjectHandleOnStack type)   at System.RuntimeTypeHandle.GetTypeByName(String name, Boolean throw , Boolean ignoreCase, Boolean reflectionOnly, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean loadTypeFromPartialName)   at System.RuntimeType.GetType(String typeName, Boolean throw , Boolean ignoreCase, Boolean reflectionOnly, StackCrawlMark& stackMark)   at System.Type.GetType(String typeName, Boolean throw , Boolean ignoreCase)

   at System.Web.Compilation.BuildManager.GetType(String typeName, Boolean throw , Boolean ignoreCase)   at DotNetNuke.Framework.Reflection.CreateType(String TypeName, String CacheKey, Boolean UseCache, Boolean IgnoreErrors)

We solved the issue in our case. I forgot to tell you that this occurred while I updated from 7 to 9
I installed a whole new dnn local and compared the folder structure on the site that failed.

It turns out that the upgrade didn’t succeed to copy all files to the bin folder.
It were missing 80% of theses marked files...

Thanks for the reply.  I have all those DLL files in my Bin folder as I made sure the zip file was unblocked when upgrading.