Database added to Dataguard configuration fails after 5 minutes

I have an existing Dataguard configuration that I want to add a third database (on a different server) to.

I have successfully added the database but after about 5 minutes I get an error message in dgmgrl, and once I get this I can no longer log into the new standby database from the primary.

oracle version 12.2

I have all the archivelogs I need on the primary server, and for the brief period that it works you can see that logs have been applied. I've tried everything I've found on google, and recreated the orapwd file and copied it round about a dozen times. I've also tried creating a new orapwd file on the standby server, unsuccessfully.