Recently Lightning stopped showing reminders for due tasks or events. I'm not sure why, but the last reminders I dismissed were for a long series of missed reminders for a single specific task, and I had a little difficulty avoiding error messages from clicking through the completions too quickly. (I've had write errors from this in the past and they were always transient.) Now, however, nothing shows reminders. I tried disabling all other addons, but that made no difference. I've exported all my calendars, deleted and recreated them, shutting down Thunderbird completely each time, and even tried copying and pasting a minimal set over from one such recreation to a fresh calendar. I used an ICS validator on one of the exported calendars, and fixed a few minor errors that way, but nothing works. Nothing shows up in the error console except warnings about Lightning using obsolete overlays (I'm using 68.12.1) and similar.