I have a problem authorising any admin tasks on Yosemite 10.10.5 (like installing new apps, unlocking system settings etc.). I have an admin account with a password, yet the system won't authorise it. The prompt just shakes around as if the password was incorrect. However, I'm able to log into my account without problems, so I assume the login details are fine.
I've noticed that the "Username" field is blank by default (while usualy it's already there when the prompt appears) - please see the attached image file. Even if I type the username manually, it still wouldn't work.
Not sure what to do about it, I've never seen anything like this. Any help would be much appreciated!