I am trying to determine if the failure is with Ubuntu or VirtualBox.

Using Ubuntu MATE 16.04 with the Ubuntu repository 5.1.38 VirtualBox.

To remotely connect I am trying to use VNC rather RDP from the proprietary extension pack.

I verified the VNC extension pack is installed, both with the GUI and in /var/lib/virtualbox/ExtensionPacks/VNC.

I created a password:

The password is always rejected as failed.

I am using remmina but I see the same failure with xvnc4viewer. An nmap scan shows the configured port is open.

Thanks.

Note: The Oracle repository and *.run script are not solutions because the VNC extension pack is not included in those versions.

This is just a way out guess. If you look at the VB section I posted in your other thread there are no quotes surrounding the secret. I would try

VNCPassword=test

Not sure how vboxmanage accepts or parses the password.

Well, this is just nuts. Connecting with 5.1.38 now works. Password or not. Quotation marks or not.

I have no idea what I might have done to fix this. This was indeed broken earlier today -- and yesterday when I first started testing. Originally I used quotation marks and the password was rejected. I don't know if the quotation marks affected the handshake. I have since tried a password with quotation marks, without, and with no password. Beats me.

Huh. Not my imagination after all. I tested on another system and experienced the same original issues.

And the problem returned on the first machine too. Nothing I do now gets VNC to succeed.