Hi @arenanoc. Welcome to GoDaddy Community! I'm not sure what would be causing this, but since you were able to install WHMCS, I would guess that you have a VPS or Dedicated Server. In that case, what you're experiencing is likely due to a configuration issue within the server. I did find this online forum discussion regarding the same topic and thought it might help. One of the suggestions was regarding your IP changing. That shouldn't happen but if it did, it would be something on our end and you could review that with our server support team.
JesseW - GoDaddy | Community Manager | 24/7 support available at x.co/247support | Remember to choose a solution and give kudos.