Why does my Task fail?

Below is a list containing questions to help debug why your task was not sent.

Do all tasks fail? Or is it only one?
If only one task fails: Are there any differences between the task failing and other tasks that send successfully?
When did the error start occurring?
If all tasks fail: Did you or the system admin change any settings recently? Keep DNS, server, or system changes in mind.
Are the Personal Access Tokens (as of version 2.0) still valid and active?
Please review the browser's debugging tools while sending or editing a task (Debugging).
Check the log files in the installation folder (Log files).

Read here how to find out how to investigate what went wrong while processing my task.