Help -- MultiMarkdown Converter Stopped Writing Files!
Normally when using MultiMarkdown Converter, you drop a text file on the window, and an HTML file (or whatever) appears next to it almost instantly.
As best as I can tell, this is related to macOS sandboxing. Briefly, sandboxing is a security tool used by Apple to limit applications in their ability to access files on your computer. In general it's probably a good idea, but a bit of a hassle at times for developers.
If you find this happening, use the "Reset Settings" menu command -- this will reset the visible settings, but will also reset the sandboxing configuration. This should allow everything to work properly again.
If you can figure out how to reproduce this effect, please let me know!
UPDATE: There is one case of this happening and not being fixed with resetting the preferences. It then resolved spontaneously a couple of days later. There is a chance that it could be related to "Default Folder X", but this is not confirmed.