Comment 1 for bug 1728143

Are you knowledgeable enough to log in remotely and attach a debugger to the gdm3 process when it is frozen? If so, please do and then paste the stack traces of the hung gdm3 process here.

But also...

Thank you for taking the time to report this bug and helping to make Ubuntu better. Please execute the following command only once, as it will automatically gather debugging information, in a terminal:
  apport-collect 1728143
When reporting bugs in the future please use apport by using 'ubuntu-bug' and the name of the package affected. You can learn more about this functionality at https://wiki.ubuntu.com/ReportingBugs.