I found the culprit: it was the UxTheme Launcher, which I previously uninstalled, but it was still loading its files (no idea why), so deleted it manually.
Now I can't remove its entry from the Startup list, but that is another issue (the top one)
Btw I was trying to patch uxtheme.dll for 3rd party themes to work, but they didn't. Are there any restrictions in Windows 8.1 for that?