Remote clients appear connected but server says they are offline

Hi, Spoke too soon. They both just dropped offline again.
This is truly frustrating. Can anyone suggest where to look.

Extract from last entry in log file
2023-04-28 02:30:42: urbackupserver: No available slots… starting new Worker
2023-04-28 02:30:42: ClientService cmd: STATUS#pw=XXXXXXN0ufvInZrIfFH0KhpMbIGmY
2023-04-28 02:30:42: Trying to connect to internet server urbackup://XXXXXXXXXXX.com.au:55414
2023-04-28 02:30:42: Successfully connected.
2023-04-28 02:30:52: Error receiving challenge packet
2023-04-28 02:30:52: InternetClient: Had an auth error
2023-04-28 02:31:02: Error receiving challenge packet
2023-04-28 02:31:02: InternetClient: Had an auth error
2023-04-28 02:31:33: ClientService cmd: STATUS#pw=XXXXXXN0ufvInZrIfFH0KhpMbIGmY
2023-04-28 02:32:25: ClientService cmd: STATUS#pw=XXXXXXN0ufvInZrIfFH0KhpMbIGmY
2023-04-28 02:33:16: ClientService cmd: STATUS#pw=XXXXXXN0ufvInZrIfFH0KhpMbIGmY
2023-04-28 02:34:04: ClientService cmd: STATUS DETAIL#pw=vc6dx0KNXXXXXXZrIfFH0KhpMbIGmY
2023-04-28 02:34:04: rc=0 hasError=true state=0
2023-04-28 02:34:08: ClientService cmd: STATUS#pw=XXXXXXN0ufvInZrIfFH0KhpMbIGmY
2023-04-28 02:34:12: ClientService cmd: UPDATE SETTINGS internet_mode_enabled=true
internet_server=XXXXXXXXXX.com.au
internet_server_port=55414
internet_server_proxy=
internet_authkey=XXXXXXXXXX
computername=EDR00MtEv
#pw=PjKpVCJH5BXXXXcqGOUfrgtWuGqHaq
2023-04-28 02:34:12: rc=0 hasError=true state=0