WowAce

What version of Grid2  are you using (Version is displayed in General/About Tab) ?

Grid2-2.8.0

What game client version, classic or retail, windows or mac and language are you using ?

_retail_

Windows 10 

Did you try having Grid2 as the only enabled addon and everything else disabled?

Yes

Please provide any additional information below.

Probably you are using  the "None"  layout when you are solo. And this is the main grid2 window without any unit frame (only the border and background decoration).

If you want to hide the main grid2 window when solo instead of using the none layout goto:

1. General/Appearance/Layout  Look&Feel

2. In the "Show Frame" option select "Grouped" instead of "Always".

Thanks for explaining - much appreciated.

However, changing the Show Frame option from Always to Grouped makes ALL my frames disappear when Solo. What can I do to ensure at least the Player and Target frames remain visible, and the Main Grid2 window is not visible when Solo? Or alternatively: Can the Main Grid2 window be relocated to a less obvious location on my screen?

You can enable themes and use two themes, one theme when grouped another theme for solo (Themes are enabled in the general tab, when themes are enabled a new themes section is displayed to create and manage themes).

Each theme allows to setup a different appearance configuration:  positions, layouts sizes, orientations,  etc, but this maybe is too much work only to hide the main window, because you have to configure some things twice.

Usually people use themes to setup one appearance for 5 man content, and another appearance for raids (with smallest frames and positioned in different place).

Usually this issue is non-existent because most people hide the decoration border, so the border is never visible.

I will dig into this issue, player/target/focus frames are not used by many people so this was never a problem until now,

 Update to version 2.8.1-beta, in this version the decoration border must not be visible if no frames are displayed.

Updating now...

EDIT:
The decoration border is no longer visible when no frames displayed. Thanks!

One last question though, Bugsack reports the following error after update to v2.8.1-beta.

Is this something I did? - I extracted the files manually because I couldn't update through the Curseforge Desktop App as I normally would. 

It seems  you copied the raw files from the github repository instead of the released .zip file.  You must install the released zip file, not the raw files.

https://github.com/michaelnpsp/Grid2/releases/download/2.8.1-beta/Grid2-2.8.1-beta.zip

Unfortunatelly curseforge did not publish the new version yet, sometimes curseforge gets slow or stucked and instead of a couple of minutes they need some hours to approve new addon versions.

Curseforge updated their Beta files to include v2.8.1. I ran the install and voila! - No more Bugsack error. Thanks again for your help on this issue. Cheers!