Data Credentials in PowerBI Service - The credentials provided for the SQL source are invalid

Hi all, 

I've seen this be a problem for several over the years but I can't seem to find a 'proper' solution.

I have a pbip file that is pulling data from a Synapse DB as the only data source. Previously my colleague had set up the Semantic Model and it was refreshing with no issue, I have needed to take the model over but unfortunately I now get the above error message when trying to update the credentials. My admin account has all relevant permissions to connect to the DB in SSMS and it allows me to refresh the data in Power BI desktop. However as soon as I publish the model to Service I get the following;

On trying to use my admin account to update the credentials I get this;

I am using Oauth2 in both Desktop and Service with organisational privacy level. I have also got rid of all global permissions in the desktop file. What's more annoying is I've left the UAT version of the model in the previous owners name and all appears to be refreshing and working. 

What am I missing? Any help greatly appreciated

Thanks