I use VTM 26 and specify the output folder c:\output together with the option "preserve folder structure". If I generate thumbnails for videos in the network folder \\diskstation\videos, then the thumbnails are stored in a folder with the strange name c:\outputdiskstation\videos. In my opinion, the expected behaviour would be to store the thumbnails in the folder c:\output\videos, since the server name "\\diskstation" should be treated like a drive letter. I guess the reason for this behaviour is that VTM expects paths to start like "c:\..." and overlooks network paths starting like "\\diskstation\...".
Best regards,
VtmFan