Zoiper saves its contacts file in 'C:\Users\your username\AppData\Roaming\Zoiper'.
Make sure that the current Windows user account has full read & write access to that location.
You can also try starting Zoiper as Administrator, to do so: right click on the desktop icon of Zoiper and select -> Run as administrator.
If the issue persists, please check for any antivirus or system cleaning software that might remove the Zoiper database or configuration files from 'C:\Users\your username\AppData\Roaming\Zoiper'.
If you are in a domain controlled environment, the issue could be related to the domain controller configuration.