Accidentally started the launcher through shortcut in the main menu.
It went on update rage and present me with an unsorted list of my profiles with a flashing, like an exalted idiot, advert.
Well, advert wasn't quite the biggest problem. The biggest problem was that it thoroughly destroyed every profile I had saved. Every attempt to start the game ended up with failure and investigation had shown that each profile I've tried to start had its custom profile folder set to empty string.
Every attempt to correct this misbehavior ended in the same result - empty string set as custom profile directory, no matter how exactly I choose it - copy-paste the full path or use dialog to select it.

Note that parent of MC profiles' base directory is a volume mount point.
If you are attempting to resolve it by hands instead of using OS API functions, you will end in the void between disks.