Unknown class info in Interface Builder file

The warning may be because you have assigned a removed/deleted custom class name for one of the UIView subclass. In order to fix such scenarios,

Open interface builder class as Source Code by right clicking on it (Open As -> Source Code).
Find your unknown class name in the source file. And handle accordingly.

It worked for me.

See my answer to https://stackoverflow.com/a/29013058/784318

I did run into this problem today using Swift.

I changed a class Model.h + Model.m to a Model.swift. This object was used in Interface Builder with the class = Model.

As soon as I replaced the object the class could no longer be loaded.

What I had to do was to change the class reference in IB from:

Class = Model
Module = 

to

Class = Model
Module = <TARGETNAME>

You'll find the <TARGETNAME> in the build settings. It is also the name that shows up in your generated Swift-Header: #import "TARGETNAME-Swift.h"

Try this, in order:

1. Product->Clean in Xcode
2. Delete the app from the simulator or device
3. Restart Xcode
4. (Build &) Run again

If this doesn't help, you likely have a reference to a class in the nib or storyboard that you have to manually find and remove.