Hi timtam
I suspect there are a combination of factors behind the cause. If the online file is (or was v2019.3) this is both an old file version and also an old database version. I suspect this is what is causing the connection error during attempts to upgrade the file version.
AR2023.5 (being the new database version v4.0) cannot connect to the old database version 3.5, and vice versa. Typically online files must be running the latest version and this file has not been accessed for a number of years. When the online file was last accessed as a v3.5 database version (preAR2020), the file was 'checked out'.
I doubt you will be able to update it to AR2023.5 online. And while this file is online, you cannot upload a new file, as under a Plus subscription you can only have one file online at a time. You can only restore. However you cannot restore, because it's status is 'checked out'. When the database version was changed to v4.0, MYOB removed the 'Check in and check out' process as it is not supported by v4.0.
The solution - contact MYOB support and ask them to remove the online file. Run AR2023, open the local file and then go File Upload. As the online file has been removed, you should now be able to upload a new online file.
Let us know how you get on with this.
Regards
Gavin