10.11: System Events got an error: Application isn’t running. (-600)

I’m getting the error

System Events got an error: Application isn’t running. (-600)

with various scripts in Script Editor and in Alfred workflows in the El Capitan GM.

even the one line script:

tell application “System Events” to activate

produces the same error.

Any suggestions?

I’ve submitted a feedback report and would urge others to do the same.

System Events is working fine here.

rebooted (for the first time since installing El Cap) and it’s now working fine here too.