Error 500 : variable is undefined VBScript Code : ValidateTSList

Hello,

I can't load wizard properly while PXE booting on a virtual machine and I'm getting this error :

Error 500 : variable is undefined VBScript Code : ValidateTSList 

This is working without any trouble on a physical machine but it's broken on a virtual machine... (we are using Hyper-V and Windows Server 2012 R2)

Do you have some ideas about this ? Scripts files haven't been modified manually... I already updated deployment share and changed the boot image in our WDS, it didn't resolve the problem.

Thanks a lot,

Valentin