Try running a systemwide update. Open a terminal and type:
sudo apt-get update
sudo apt-get upgrade
Reboot and try to log in again.
Which version of Firefox? Which version of iOS? If not the latest version (of either), can you update and try again?
If you’re using Chrome, the issue may be related to this one:
Please try accessing with another browser.
And let’s wait for @anoek. :/