I have no idea why but my notification popups suddenly stopped working.
They appear in the notifier widget, but there's no popups at all.

At first I assumed I had changed a setting in Desktop Effects, but the settings are identical to another machine I have set up the same way, and popups work fine on the other machine.
Any ideas?

cat /etc/issue
Ubuntu 14.04.5 LTS \n \l

uname -a
Linux betlognuc 3.13.0-98-generic #145-Ubuntu SMP Sat Oct 8 20:13:07 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux