Try to Reset AutoCAD: menu AutoCAD LT -> Reset AutoCAD.
Then try to reproduce your steps (saving at last stage).

Plus:
You have not answered other questions:
- Do you have issues with saving drawings with only a few lines in it?
- How long ago was this BEFORE? Have you installed some system updates recently?

And here another one:
If you are able to save drawing during editing till some moment, what are the last steps after which AutoCAD freezes on save?