I'm using Alfred 4.0.6 on macOS Catalina (10.15.1). Every few days, the toggle stops working. I can still bring up the Alfred menu item and open preference, but neither the toggle option on the menu item nor the toggle keystroke (ctrl-space for me) works. The only way to get it working again seems to be to quit Alfred, then restart it.
I'm not sure what other useful information I can provide would be useful, but I'd be happy to provide what I can. I did check with