Suppose you have a processor with 2*n cores running one worker using 2*n cores.
If you change to two workers, you must change the first worker to use n core(s). The error message leads me to think that you changed the number of workers, without adapting the "CPU cores to use (Multithreading)" setting in consequence.
The total of the number of cores used by all the workers must not exceed the number of cores (or the number of threads in the case of a processor supporting hyper-threading.)
Jacob
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