Getting "Cannot verify client. (3000)" when trying to upload archive from Xcode to itunesConnect

Hmmm, well a restart of Xcode, a fresh clean, build and archive seems to have fixed it.

If you encounter this error, you have two options:

Option A - Engage in an activity that takes a minute or two. Could be any of the activities described in the other answers, like re-install XCode, renew certificates, do a new build from scratch after clearing caches. Or simply go get a coffee.

or

Option B - Do nothing at all for a little while.

Then, try again. If it doesn't work, repeat.

Just updated to Xcode 7.1 and getting this error. Was able to upload fine using 7.0.1

same here - restart, clean build, archive and upload was successful.