When that dialog opens, it's pretty much out of our control. However, various types of software that tries to "enhance" your PC experince (such as Dropbox, Antivirus software, etc.) and install so-called Shell Extensions which are loaded both into Explorer (e.g. to show Dropbox sync status) and also any file dialog of any application you're using. It sounds to me like one of those shell extensions is doing suspcious stuff. The fact that Explorer starts hanging in a similar way afterwards is another indication that you have one of those shell extensions installed and it's misbehaving. OpenMPT itself certainly cannot hang Windows Explorer during the splash screen, because it doesn't interact with it at all. Your best bet is finding the broken shell extension and disable it.

Closed as there was no feedback and most likely nothing we can do about it.