I have never seen the clear ip policy-sessions command cause an issue like that.
Did you reboot the router at all before you saw that?
I believe my 3458 rebooted the other day when I did a clear ip policy-sessions,
however if came right back up...
I couldn't prove one way or another though...
When issuing the "clear ip policy-session", it will shut down all sessions that are open on the router. This includes the session you may be using to access the router. You should be able to log right back in.
However, I have not heard of any issues with the "clear ip policy-session" command causing a reboot. Based on the output provided, it looks like the firmware images were not found when the router booted back up and seems to point to a FLASH issue if anything.
I would suggest upgrading to the latest maintenance code which can be found here: Software Downloads and contacting Adtran Tech Support if you see this issue again.
Please do not hesitate to let us know if you have any questions.