Unable to enter safe mode. Gets stuck at progress bar (100%) while trying to log in Macbook Pro Retina 13", late 2013.

Unable to enter safe mode (via holding shift key). Result as in title.
Hangs after me waiting for mutliple minutes.

What troubleshooting can I do?

Need safe mode to do other troubleshootings.

Edit1: note, that the progress bar at first moves but very very slowly, compared to non-safe-mode logging-in.
Maybe I should wait overnight or something?
Is there a way to show any debug messages while this is running?

Also asked on Apple forum: https://discussions.apple.com/message/31699416#31699416
The "solution" was just to wait much much longer (longer than any other boot you have seen), patiently and not assume it was broken :).