Had this same problem. In my case it was a conflict between Jetpack and W3 Total Cache.
To get access to your dashboard back use your FTP client to get to your site. Go to /wp-content/plugins/jetpack/ and change the name of the folder to something like /jetpack-temp/
Then you will have to go through trial and error to see which plugin Jetpack is conflicting with. Be sure to change the folder name back to /jetpack/ before you activate that plugin again.
If I have to choose between Jetpack and 3TC I’ll be kicking Jetpack to the curb.
Yes Jetpack has messed up my website. When I logged into my admin panel this morning I was informed that Jetpack needs an upgrade. As usual I went through the normal upgrade process, but then all I saw was a blank page with the message : Briefly unavailable for scheduled maintenance. Check back in a minute.
Horrors!! The website could not be accessed at all. Finally I went into my webhosting cpanel and deleted the Jetpack folder. Went back and cleared the browsing history and refreshed the page a few times,and my website came up again.
I went back to my website admin panel and saw the message : “An automated WordPress update has failed to complete – please attempt the update again now.”
NO THANK YOU. Anyway I reinstalled Jetpack and activated it. But I find that it is still messing up some pages on my website. Note that Jetpack has had no conflicts with any other plugins on my website before this. The only change now is the Jetpack upgrade to version 2.0.
What I’m doing now is deactivating Jetpack and uninstalling it. I’d rather wait for the Automattic guys to be alerted about this and do something to resolve it,
