We were finally able to reproduce this on the Imager 2.0 release. The Windows PE ISO will fail to build if Imager is run from a UNC share, with a warning in the log file (under %LOCALAPPDATA%\VMware\Imager\logs) similar to:
[WRN] '\\machine\share'
CMD.EXE was started with the above path as the current directory.
UNC paths are not supported. Defaulting to Windows directory.
A fix is planned for the next release. For now the workaround is to simply don't run from a UNC share. Mapped drives seem to work fine.
I keep failing on Prepare, just keep getting Failed to create customized Windows PE ISO.