My N9 started to act strangely yesterday evening; it just never connected to either Wifi or 3G. A reboot would certainly help... or so I thought.

Right after I rebooted, I got a screen telling me (in German) than my security code has expired. I'm asked to enter the old code and then a new one (2 times). The new one must be at least 5 characters long and must consist of letters and numbers.

I did read the threads about people suddenly being asked to enter a device lock code that they never defined, and if it's really necessary I'll flash my device... (if I still can. Are the necessary files still available for download from a trustworthy source?)

But I'm not even sure what "security code" this is. The thing about both letters and numbers doesn't sound like a normal device lock at all, neither does the word "expired". (Device lock codes don't expire, do they?)

One thing I could think of is Mail for Exchange. Our company enforced provisioning quite a while after I had set up the account, and while the N9 did ask me at some time (cannot remember when) to enter a lock code, it never asked me again to enter it when I switched it on. (Is this a bug or a feature in the MfE implementation?) Because of this, I'm not 100% sure what I entered back then, but I'm pretty certain it was the PIN I also use for my SIM card... which, of course, doesn't meet the standards required now for the new code.

So before I go and wipe my device and re-flash it, I'd like to know where this "expired security code" really comes from and if I can bypass this message in a less destructive way.
(I don't know, like... like uninstall MfE altogether if it's really an MfE issue? Is this even possible?)

UPDATE: I found the mail from Oct 2013 announcing Exchange Provisioning; they said it should be a 4-digit numeric PIN. I'm pretty certain now that I used my SIM-PIN. That makes it less likely I'm dealing with an Exchange problem here. So where does the "more than 5 characters alphanumeric security code" come from???