If an ad-hoc task fails during execution and leaves an open lock behind, the next run of this ad-hoc task will fail with an exception and the following error message:

!!! Error: Call to a member function get_fail_delay() on null !!!

Reason: If obtaining a lock in core\task\manager::get_adhoc_task() fails, this function will return null and core\cron::run_adhoc_task() tries to call get_fail_delay().

This happens if you start the same taskid twice from the CLI.