Reminderfox up to 2.1.6.X doesn't run on Thunderbird version greater than version 60.* (60.9.1 would be OK).
If your system was automatically updated to TB version 68 and it seems you have lost your reminders/tasks .. don't worry those are on your system without change. You need to reinstall Thunderbird 60 and make sure it's not updating to version like 68.
1. Find your OS and language TB version here https://releases.mozilla.org/pub/thunderbird/releases/60.9.1/
2. Download and install your selected version (after you closed TB before)
3. Start TB and on the main menu open 'Edit' --> 'Preferences' (maybe different on WIN or MAC)
4. Select the tab 'Advanced'
5. There you check the selection "Never check for updates"
So you have installed TB 60.9.1 and your current Reminderfox with all reminders/evnets/tasks etc should be back.
Reminderfox has a new home for the documentation neandr.github.io/reminderfox
This change was necessary because the pages of reminderfox.org were closed.
The new home contains English and German documentation.
The German documentation is called with the application (Thunderbird)
installed in German.
All other language installations show the English documentation.