However, Tasks doesn't use ESource yet and people had noticed that if you add items to Tasks, they don't appear in Evolution or the panel. This is a major problem, so I build EDS and debugged the problem. Turns out that the system tasks store is at ~/.evolution/tasks/local/system/ but when using ESource it is at ~/.evolution/tasks/local/system (note the lack of a slash) and as backends are looked up using literal string comparisons, these don't get the same backend so you don't get change notifications, and even worse modifying the task list with one application will overwrite the other's changes.

Luckily the fix was trivial, and I hope that it gets reviewed soon. Update: committed!