Error adding a file in the Customization Project Editor within any PORTAL site

Kia Ora all,

The ability to click the plus sign for 'add a file' within the customisation project editor when you are logged into the PORTAL stopped working a while ago.  This happens on any local OR hosted instance of any site (client or demo) on which we have tested. 

An up to date Acumatica instance works fine. 

The functionality in MYOB Adv used to work fine prior to at least Build 2021.111.201 (which throws the error).    The functionality works on any version, any instance, local or hosted perfectly from the MAIN site - just not from within the attached Portal site from around 2021 onwards in MYOB Advanced only.

The error message is simple "An item with the same key has already been added".     

A high priority ticket has been logged for a very long time as the client is unable to be upgraded due to this issue.

Does any other partner or MYOB Direct consultant have a client stuck with this same issue - or has tried to customise the portal for a client and found they cannot do so using the Add File function anymore/at all? 

Any suggestions / workarounds/ other comments greatly appreciated. 

Cheers!!