Half an hour ago it did not work, then after the reboot it worked once again, since then it does not work at all anymore.
So I agree it can actually and only be number 1.
I’ll now try once more getting the Recovery (or Rescue) Console via CTRL. If it doesn’t work, lucky me I found an image backup from June this year and another class 10 64 GB memory card 
So going to the console locally via Exit → ESC and then try to login is also not working?
nope, but I can already report that the account lock phenomenon even occurs with Kodi 16.1 (from which I still found the image) and another SD card. Weird. I never had such an issue before.
I am now trying to reproduce if that comes from Total Commander, after trying to use it via SSH to browse the KODI file system.
Up to now everything works fine.
After 5 attempts or so I only had the logon deny issue once, but after rebooting the Pi all was well again.
Will try with the previous SD card and the latest Kodi version once more and then get back to here.
No any issues anymore. Let’s hope it stays like that