This is what I see in the dask-worker output.
[2020-02-07 20:13:43,099] INFO - prefect.TaskRunner | Task 'retrieve_url': Starting task run...
objc[68582]: +[__NSCFConstantString initialize] may have been in progress in another thread when fork() was called.
objc[68582]: +[__NSCFConstantString initialize] may have been in progress in another thread when fork() was called. We cannot safely call it or ignore it in the fork() child process. Crashing instead. Set a breakpoint on objc_initializeAfterForkError to debug.
distributed.nanny - INFO - Worker process 68582 was killed by signal 6
distributed.nanny - WARNING - Restarting worker