Hi Bastian,
I recently manage to solve this. This is definitely a bug with the lock system and not the encryption.
To disable the password lock feature, edit the global.mk file in /opt/omd/sites/YOURSITENAME/etc/check_mk/multisite.d/wato
lock_on_logon_failures = 5
Set the value to 0, reset your password with htpasswd -m htpasswd cmkadmin and you should be able to log in.
Once logged, you can set another password and maybe setup a rescue user, then set back the lock feature from the admin panel - Setup / General / Global settings - at the bottom of the page
Hope this help and that the devs will file a bug.