That also happened to me and I got it working with these steps:

1. Uninstall Amethyst including cache and storage
2. Change socks5 port 9050 to anything else in Orbot
3. Open Amethyst and connect to Orbot with the port number you changed
3. Open Orbot and exit Orbot from More tab
4. Launch Orbot again, and connect to Tor network
5. Open Amethyst and refresh page