Sending Identity to client "CLIENT" failed. Retrying in 60s

I am getting the error below on the server when trying to configure a client

2021-07-30 12:25:55: Authed+capa for client 'CLIENT' (encrypted-v2, compressed-v2, token auth) - 1 
spare connections
2021-07-30 12:25:55:  Sending Identity to client "CLIENT" failed. Retrying in 60s.

There is not a whole of of useful information in the client debug log either:

2021-07-30 12:25:35: Trying to connect to internet server "x.x.x.x" at port 55415
2021-07-30 12:25:35: Successfully connected.
2021-07-30 12:25:41: ClientService cmd: STATUS DETAIL#pw=thisistheencodedPW
2021-07-30 12:25:41: rc=0 hasError=true state=0

C:\Users\User>netstat -an |find "35622"
  UDP          *:*
  UDP    [::]:35622             *:*

C:\Users\User>netstat -an |find "35623"
  TCP              LISTENING
  TCP        TIME_WAIT
  TCP        TIME_WAIT
  TCP    [::]:35623             [::]:0                 LISTENING

C:\Users\User>netstat -an |find "35621"
  TCP              LISTENING
  TCP    [::]:35621             [::]:0                 LISTENING

The server is an internet mode ubuntu server.
The client is Windows 7. There is no local firewall enabled and no restrictions between the client and server (apart from the local router doing NAT to the internet).

I don’t really have a whole lot to go on here as the logs are pretty sparse. Anyone any ideas?

Bump… having this problem on more than one client now.

Can anyone help me with the same problem?