There is a bug in how 'Flash Memory' activates on a running system. If Notifications are enabled (postfix) when Flash Memory's does it's startup mounting/copy, it corrupts postfix. I'm guessing it is occurring because Flash Memory is moving around files that are actively being used by postfix.
In the guide https://www.debian-administrat…/661/A_transient_/var/log, it recommends doing this at start-up before anything is active and using the files. I've also confirmed that if I disable 'Notifications' before enabling 'Flash Memory' this issue doesn't occur.
I think the safest course of actions would be for 'Flash Memory' to not try to enable itself immediately on a running system, but to wait and enable it on the next reboot.
To replicate:
1) Enable 'Notifications'
2) Enable 'Flash Memory'
3) Check the status of postfix (postfix -status) or try to mail something. For me the mail queue froze and wouldn't send anything.