Hi Paul,
I am experiencing an identical issue to Tomasz on my staging site where I am testing the Helix update. I am currently running Joomla! 3.10.2, and I updated Helix from 1.1.2 to 2.0.5. I am not using jsecure, I have SSL enabled, and I tested this in multiple browsers where I got the same results (again, nearly identical to the image Tomasz sent). Hopefully that information is helpful to you as you continue testing.
I would appreciate any update on this issue if you are able to find a solution.
Thanks in advance!