I had the same issue a while ago and I updated my graphics card driver and it solved the problem.
Do blind typing in the respective username and password field it will log u in
This is a know issue of mx 9.6
Changing your screensizing helps
This issue is still present in Studio Pro 9.12.5. In my case I have a laptop screen and an external screen. The username field stays empty when the dialog is on my laptop screen, but when the dialog is dragged to the external screen, sometimes the characters appear. It probably has something to do with the scaling settings (125% on laptop screen and 100% on external screen).
I don’t want to give up...but I can’t solve this same problem with 9.17.1. My graphics card driver is the most updated. I tried scaling settings from 100% to 175%. It doesn’t work. It is too challenging for a non-IT background people as a beginner...sigh