By default, NETDDE applications are limited to 16 connections. When the application attempts the 17th connection, it will fail and post an error to the event log.
You can set the number of connections by navigating to:
Edit or Add Value a type REG_DWORD named MaxSessions and set it, using the Decimal Radix, to a data value between 0 and 254.
A data value of 0 defaults to 16, as does a missing entry.
You will need to reboot.