Error 408 is shown when the server was unable to produce a response to your register request within the suitable amount of time. You may need to disable or enable STUN and rport in different combinations according to their current state e.g. if they are enabled try disabling them and vice-versa. You will find these options in Config-> Accounts -> Your registered account -> Network Settings.
Please also make sure that the default ports used by Zoiper are not blocked from your firewall or routing device. The ports used by Zoiper are as follows:
SIP port is 5060
IAX port is 4569 UDP
RTP port is 8000 and above UDP
Default STUN values are:
Server hostname/IP: stun.zoiper.com
Port: 3478 UDP/TCP
Refresh period: 30
If you keep receiving this error after that, you will need to contact your VoIP service provider for further assistance.