A few things come to mind:
You have DSLstats installed in a folder at the top level of the C: drive, and from time to time users have reported problems with this, which go away when it's installed in one of the user's own folders (under Program Files for example). If you had v5.8 installed in the same way, then this is probably a red herring, but if not then it might be worth moving it.
The fact that v5.8 worked well suggests that the problem relates to changes made since then. I'll investigate this, but nothing leaps out, and several people are using v5.9 on Windows without issues.
Someone who contacted me via my website with a somewhat similar issue found that reinstalling DSLstats and deleting its configuration files before restarting fixed the problem. The configuration files are by default saved in the AppData folder, which is the same as where snapshots are saved (unless you've changed this).