Had the same problem that everyone else was describing when I had done a fresh install of 7.6 on a 2012 R2 server.
Fixed my issue by going to Options / Security / Users and Authorizations ... and clicking the "Authorizations" tab. There, I found my initial default user ("Administrator") with Full Rights Toolbox to CMS systems, but there was also an entry for the same user with "Limited Rights" Toolbox to the same group. I clicked the box at the beginning of the line that granted the first user to the "Limited Rights" Toolbox and DELETED it. Now I can setup e-mail alerts/notifications with no error.