When I start the launcher everything behaves as expected. I can access all settings, customize my profiles, can access my skin settings. And I can launch everything I want.
But the moment when I switch accounts and try to start anything the launcher crashes. Every single time.
It doesn't matter if I'm using the same game directory or a separate one, it doesn't matter if I'm using an upgraded environment or simply start the whole thing from scratch, keeping the launcher open or not makes no difference, it makes no difference if I'm using snapshots or advanced settings or not, the account doesn't matter and most of all I can't find anything specific in the logfile (attached).
The second run it briefly mentions "Preparing", then Windows tells me the launcher crashed (twice) and that's it. I can restart the launcher without problems, the moment I try to launch Minecraft it will crash no matter what.
When I stop the already running Minecraft game and try again everything works as intended once more. Until I try once again to start another copy of the game using a different account.
For the record: the old launcher never gives me any issues. After this ordeal I grabbed it from my backups and have been using it ever since. I figured I'd wait a few weeks before filing a bug report but a quick test today showed me that nothing changed: I can't start Minecraft using multiple accounts with the new launcher.
As mentioned in the bug report itself I've avoided the new launcher and continued to use the old one, (very) sporadically trying the new launcher again. Well, today I got upgraded to 2.0.895 and this problem has been fully resolved.
So this issue can be closed, I'll see if I can do this myself somehow or if I can report it.