I have the same problem. I am on Android Lollipop 5.1.1 and no web-browser will display the 3cx Web Interface. All I get is the spinning circle. I have tried the stock browser that comes with Lollipop, CM Browser, Opera and Firefox. None of them work
My workaround is to use the Microsoft RDP Client App on my phone to remote into the 3CX Server and then use the Web Interface on the Windows machine.
Wow, that does work on Android Chrome. Now that I know that I am sure that I will never again use it.
I presume you are using an Android giant device like a tablet of some sort. I wouldn't want to do any work on a screen the size of my phone. My Surface Pro 3 works great for this.