Sometimes, especially when the "Record Document Mappings" option is enabled, the SQL Express database will reach its 10gb limit. The size of the database files can be seen in C:\Program Files\Microsoft SQL Server\MSSQL14.POWEREDBY\MSSQL\DATA
To get around this a full version of SQL should be installed. In this case an in-place upgrade on the primary server can be run by installing SQL Server Standard or Developer editions (not a cloud based version) and choosing the Upgrade from a previous version of SQL Server option from the installer:
You can choose Custom install in order to select the Express database to be upgraded. The original databases should first be backed up.
If after the upgrade, the secondary servers are still not connecting:
- Go to SQL Server Configuration Manager
- Open SQL Server Network Configuration
- Click on your Named Instance
- Ensure TCP/IP Enabled
- Open TCP/IP
- Click on IP addresses
- Add Port 1433 to all IP's
- Restart MSSQL service