It looks like a problem indeed and to be honest, i don't have any idea what is causing this.
A couple of users mentioned this issue before but this is very seldom.
What if you use the QuickDownload action (which does not open a SaveAs dialog) ? Do you still get this error ?
No EPIPE error. I have checked this for formats WEBM, MP4, 3GPP and MKV and the download using the CoApp worked as expected.
What if you launch the LocalConvert operation (open the VDH main panel, pass the mouse over the "..." sign at the bottom left, then click on the Convert Local Files icon). This should open a file dialog. Do you also get the error ?
If I do that I get the EPIPE error.
From your screenshots, the only unusual thing i see is that your user name contains a space character. Maybe there is a glitch there. Is it possible for you to try from another user account that does not have this characteristic ?
I created a new account "Test", tried to download a Youtube video with downloadhelper and got the EPIPE error.
The EPIPE error does NOT show-up on a 32bit system running Windows 7, 8.1 or 10.
The EPIPE error does NOT show-up on a 64bit system running Windows 8.1 or 10.
The EPIPE error DOES show-up on a 64bit system running Windows 7.