Android Studio: Error parsing XML & URI is not registered

Question

After adding a new Activity to my project I get following error while compiling the layout

Gradle: Error parsing XML: not well-formed (invalid token)<

Answer 1

For me, the problem occured when I created my own subfolder for old code.

Specifically, Android Studio threw an error for all layout files that weren't in the default folder ../res/drawable/, but instead placed in my own subfolder ../res/drawable/backup.