Installed 3.9 on Ubuntu 20.04.
Banging my head on a brick wall trying to get 'nxdbmgr init' working. Testing and testing and testing credentials, delete and recreate database, etc.
Eventually realised that mysql.ddr has been replaced with mariadb.ddr.
Please could references to mysql.ddr be replaced with mariadb.ddr, eg:
https://www.netxms.org/documentation/adminguide/server-management.html
More to the point, the Debian packages ship with a config file that says mysql.ddr.