Comment 1 for bug 1628673

Same problem with Arch and Gnome 3.20, even with all the zeitgeist-related plugins disabled.
This workaround was working most of the time:
https://bugs.launchpad.net/synapse-project/+bug/1401909
However, synapse would crash occasionally so now I use a systemd user service that restarts automatically. The service cannot be enabled because systemd isn't noticed when gnome is started. Instead a .desktop file starts the service:

> ~/.config/autostart/synapse-systemd.desktop

[Desktop Entry]
Name=Synapse
Exec=systemctl --user start synapse.service
Encoding=UTF-8
Type=Application
X-GNOME-Autostart-enabled=true
Icon=synapse

> ~/.config/systemd/user/synapse.service

[Unit]
Description=Synapse launcher

[Service]
ExecStart=/usr/bin/synapse -s
Restart=always
RestartSec=20

Don't forget to uncheck Startup on login in the GUI.