WP Dashboard and Post issues

I've just noticed some weird stuff happening inside my WP installation in the past 24 hours… the top bar keeps flashing when I type and now I'm seeing this code when viewing WPMU DEV plugins: https://www.dropbox.com/s/avqp92gzr18nfbw/Screenshot%202018-05-12%2014.57.05.png?dl=0

Help!