I don't work with Sage 300 my experience is with Sage 300 CRE.  However, my understanding is that Sage 300 uses SQL for its database as does Sage CRM.  I don't know how Sage 300 is connected within the system.  Therefore, I am making a suggestion on what makes sense to me.  :-)  If I had this issue with an SQL database after an upgrade, I would review and resave the connection.  I have to do this sometimes after an upgrade with ODBC connections.  You may need to reboot or do an IISreset.

I would make my way over to the admin area, then to Advanced Customization-->Tables and Databases.  I would then open the connection (If that is the way Sage 300 is connected) review it and save it.  If there is an issue, you will get an error and that may help resolve it.  You can also check the logs.  Again, I don't work with Sage 300, I am just suggesting based on an SQL connection to an SQL connection.  :-)

Hope this helps!! :-)

I also meant to ask if it was possible that during the upgrade process a password was modified?

Yes I had to change the Sage 300 V2023 password to something complex but I updated it in CRM