How can I fix 'com.android.systemui has stopped'?

[AC Question]

'com.android.systemui has stopped'

I recently updated to Android 6 on my htc One M8. Everything appeared to be working well for a day or two, until the phone powered itself down just now.
When I tried to turn it back on, reboot seemed excessively slow, got msg 'optimising apps', followed by 'Unfortunately process com.android.systemui has stopped'.
I've tried numerous reboots, but same msg each time.
Anyone know what this is, and how to correct it?

 

[Indinisbet]

Ugghhh I'm having the same issue since the android 6 update.

So far all the fixes online involve you ACTUALLY being able to get into your phone.

Unfortunately, this makes kind of a lock screen loop upon booting, so you legitimately can't do anything.

In my case, it also stops my phone from accessing the mobile network, so I can't even do a remote wipe :/

My phone has become completely useless. I hope this bumps the thread and somebody actually has a fix!