Finally got it working. After some digging in the form, I found this post -
https://forum.playphoenix.online/server/support-center/5468-solution-thread-launcher-login-and-startup-issues#p82414
For me, solution 2 worked.
Login Error: Something interesting happened.... Error 500
When: Phoenix launcher login
This error indicates an issue with a corrupted Phoenix file or insufficient permissions accessing/editing it.
Solution 1
1. Right click the Phoenix symbol (phoenix.exe)
2. Select "run as administrator"
Solution 2
1. Press the Windows key + R
2. Type in: documents
Hint: You may also navigate to the "Documents" folder of your Windows user manually.
3. Delete the ".phxlauncher" and ".phxlauncher.ACCOUNT" files (*)
* ACCOUNT stands for your game account name - in my case it would be ".phxlauncher.ashok