FileNotFoundException when upgrading from Unity 5.4.2 to 5.5.1

I just updated to Unity’s new client and opened an existing project. It asked me if I wanted to update the code to the new version, to which I said yes. Upon opening the project, the console was full of errors such as:

“FileNotFoundException: Could not load file or assembly ‘Assembly-CSharp-firstpass, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null’ or one of its dependencies.”

Anyone see something similar when upgrading to the latest version? Or perhaps with an upgrade in the past?

same case here, no clue