Dynamics CRM Solution Import Failure - Root Components Insertion Failure 0x8004F018

I hope someone can help, this is my first question ever because I've never been as stuck as this before!

I have an on premise QA server running Microsoft Dynamics 2015 CRM running on Windows Server 2012. It contains approx 30 customisation solutions from various previous sources unknown to me.

After installing ADX Studio Solutions, the system becomes modified and overwrites some/all of the previous customisations back to defaults. The symptom is that Entity names and fields etc are reverted back.

So it is required to re-install a previous backup of the solution after installing the ADX parts. This solution is exported as a "Default Solution" prior to installing ADX Studio in a file called "Default_1_0.zip".

The problem is that the import fails when trying to import the backed up solution after having installed an ADX Studio solution.

The backup comes from the same server Dynamics instance, it's not being moved anywhere.

The process is:

1. Backup entire "Default Solution"
2. Install ADX Studio and and "Basic Portal" solution components
3. Import previous backed up "Default Solution" to re-apply the changes overwritten by ADX Studio.

This is a screenshot of the import screen showing the failure:

The log file xml has a "Solution" and a "Components" section. The Solution Section looks like this:

Solution    
Name    Default
Display Name    Default Solution
Description Solution that contains all components in the system
Version 1.0
Package Type    Unmanaged

Publisher   
Name    DefaultPublisherCLIENT
Display Name    Default Publisher for CLIENT
Description Default publisher for this organization
Email   
Website 
City    
Country/Region  
Street 1    
Street 2    
ZIP/Postal Code 
State/Province  
Phone   

Status  Failure
Message An error has occurred. Try this action again. If the problem continues, check the Microsoft Dynamics CRM Community for solutions or contact your organization's Microsoft Dynamics CRM Administrator. Finally, you can contact Microsoft Support.
Progress [%]    99.96
Duration [s]    730.9

As you can see, it gets to 99.96%.

The "Components" section contains one error on line 2781 that looks like this

Root Components Insertion Failure 0x8004F018 This item is not a valid solution component. For more information about solution components, see the Microsoft Dynamics CRM SDK documentation.
Dependencies Calculation                        Unprocessed

All previous rows are logged as "Processed".

I am in no way a Dynamics or ADX Studio expert so I'm trying to find out if anybody has any answers which may be relevant.

Some more details:

Obviously I've tried searching for answers and also tried using the crmdiagtool2011 tool, which has not yet proved useful to me.

I've also tried implementing this solution: social.microsoft.com/.../crm-2011-import-failure

But the error code and message is slightly different and this solution did not work for me.

I have managed to overcome a previous issue with importing the solution which was very similar to this one:

www.magnetismsolutions.com/.../Dynamics-CRM-2015-Solution-Import-Fails-with-Unable-to-find-Metadata

Their "option 2" involved manually editing the solution.xml and customizations.xml files of the backup which I did and it seemed to overcome that particular problem. I didn't know how to achieve their "option 1" so that wasn't an option for me.