Since two months I have to reenter credentials on random occasions since Zoiper forgets the registration.
Are you loosing your Zoiper premium activation or your SIP account settings ?
I'm having the same issue. It keeps on losing the Pro registration. So on startup, I have to put in my pro account over and over again.
Try updating to Zoiper 5.3.5, if you are using 5.3.3 there was a known issue with it and the activation.