I can suggest that you try disabling the presence by going to Zoiper -> Settings -> Preferences -> Accounts -> your account -> advanced -> disable the "Subscribe presence", "Publish presence"", and "Voicemail Message Waiting Indicator (MWI)" -> MWI must be set from the drop-down menu to "Disabled".
After disabling the above three, shut down Zoiper and wait for a few minutes. Then start it again and see if it will be able to register.