I got the same problem. Contacted HTC and they told me to hard reset, which I did. First shutting down again was possible, but two days later.. just the same problem again
Could it be an app is causing this?
Just resetted the password for "use secure credentials" (other name in my language, but i found the option) and turned it on. Stupid to see this is working. It seems that as long as I use "turn-off" holding the button and getting the normal options keeps working. This is of fastboot keeping all the information I think?
Rebooting takes ages, not like before the update, and of course the problem is there again.