Steps to reproduce:
1. Launch Editor
2. Go to Unity / Manage License
3. Attempt to activate Personal Edition
4. Select that your company earns more than $100.000 per year
5. Observe that "Please upgrade to Unity Pro" window is shown
6. Click on "purchase the Professional Editor" link
7. Observe error - page is not loaded

url: https://store.unity3d.om/?utm_source=unity&utm_medium=license&utm_campaign=serial
Unity version: 5.3.3p2, issue is also present on 5.3.0f4
OS: OS X 10.11.3
Browser: Safari, Chrome

Workaround: go to store.unity3d.com and purchase Unity Pro edition.