UnityException, material is read-only

[arvz]

Hello,

When I try to import a tmx file I get an error:

UnityException: Assets/Tiled4Unity/Materials/***.mat is read-only
Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation
ImportUtils.ReadyToWrite()    Assets/Tiled4Unity/Scripts/Editor/Importert/ImportUtils.cs:126
ImportTiled4Unity.CreateMaterialsFromInternalTextures()    Assets/Tiled4Unity/Scripts/Editor/Importert/ImportTiled4Unity.Xml.cs:132
ImportTiled4Unity.ImportBegin()    Assets/Tiled4Unity/Scripts/Editor/Importert/ImportTiled4Unity.Xml.cs:59

Running Unity 2018.1.6. Also tried it with older versions of 2018.1

Strangely if I import the same tmx & spritesheet file into an empty project, everything works fine, so it seems there's something specific to my project preventing this from working, but I can't figure out what it is. I've tried deleting every other file in my Assets folder but it still won't work!