Thanks wulilio. It looks like something is modifying Spine's files. Spine continues to work until it needs to load some code that hasn't been run yet. When it does that, Spine crashes because of those modifications. When you start Spine again, it checks its files, sees they have been modified, and downloads a new update.
Are you running antivirus or anti-malware? Those programs can be pretty nasty. You could try whitelisting the Spine installation folder and also this folder:
Windows: <user home folder>\Spine\
Mac: <user home folder>/Library/Application Support/Spine/
Linux: <user home folder>/.spine/
