It’s not an issue with the multi-core/cluster environment.
I’ve tried the suggested code, and also simply commented out the cl=my.cluster line – still no success.
However, running it on a single core gave a clue which might be helpful for debugging this: In my case, 8 models need to be fit. That step seems to work, as all eight “progress dots” (the [........]) are there and the error message pops up when the function proceeds to calculate the p-values, i.e. just after