What could this OptLink Error mean?

[Dukc via Digitalmars-d-learn]

I was building one example of DLangUi (dub build 
--build-mode=allAtOnce --build=debuglinker) and it did compile 
but the linker started to complain. I did notice it seemed to be 
related to the standard library, so I updated it and DRuntime to 
the lastest master, rebuilt them and replaced phobos.lib with the 
result. I also removed the binary and .dub directory from DLangUI 
to force Dub to rebuild it, which it did. Still the linker 
complains, but about a different symbol:

..\..\lib\dlangui.lib(colors)
 Error 42: Symbol Undefined __d_switch_string
Does anybody have an idea what could be causing this?