Not sure if this is the right place to report problems with todo.tchncs.de, but here is my issue:

I created a project and task, with start date, end date, due date, notification yesterday. Today I tried to mark the task completed. However, the checkbox does not stay in "done" state and the task goes back to the "incomplete" state. Also, I get a green popup on the bottom momentarily with the message:

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I did try this on a private window as well, so clearing cache wouldnt help I suppose