This was really hard to troubleshoot. We found that the cause of the problem was a segmentation fault in webkit (the HTML rendering engine used by MDM) affecting predominantly users of nVidia drivers. Ubuntu/Mint use an old version of libwebkitgtk-1.0-0 (1.10.2). Upgrading to a newer version (2.0.4) immediately solves the issue and the HTML greeter (which name is mdmwebkit) no longer crashes.
Now, that's the analysis.... what you probably want to know is how to solve the issue on "your" computer.. and this is where it gets tricky. Basically you need to either stop using mdmwebkit (either switch to the GTK/GDM greeters or switch to LightDM), or upgrade libwebkitgtk-1.0-0.
Upgrading libwebkitgtk-1.0-0 is a little bit tricky because it relies on other libraries which need to be upgraded as well. Here's the simplest way to do so. Beware though: You will be upgrading at your own risks:
- sudo apt-add-repository ppa:gnome3-team/gnome3
- apt update
- apt install libwebkitgtk-1.0-0 --no-install-recommends
If you're not experienced with PPAs and dpkg/apt troubleshooting, please don't upgrade anything and simply select "Menu->Login Screen" and change the style to GTK or GDM.
Thanks for posting that data . . . obviously it was complicated as it seemed to take a very long awhile to get this info . . . although the "change to GTK or GDM" was mentioned a awhile back . . . . I would volunteer to test out this terminal command, but I've taken down the LM disk to upgrade to newest OSX . . . . I know that the various problems with the greeter app were not entirely solved by going to GDM, as I still had to Log out before I could restart the computer. Hopefully someone who is still having issues and stayed with LM will try out your suggestions and will report back.