It is a Bedrock Dedicated Server - I doubt it's anything on the player's end as they are able to join other servers running on the same machine, or join to the same server through a different Xbox Live account.
I'll try updating it now and see if it helps with the problems.
We're running it on Windows 10, version 1.16.1 - we've got multiple servers hosting from a singular machine, and players are able to connect to the other servers even if joining to one doesn't work.
The server has plenty of spare resources, it's definitely not anything to do with that.
Updating the server version did not help with the issue, and it has now started to affect other servers running on the same machine.