Non [a-z0-9/._-] character in path of location: main:main/root 3

Whenever I try to boot this world, I get this crash: 

Non [a-z0-9/._-] character in path of location: main:main/root 3

The crash report is attached below. I've worked out that it's a problem with main:main/root 3 but I don't know what or where that is

Crash report attached below