Hi @Roger_Huerlimann,
Congratulations for running into one of the more cryptic error messages that dada2 can throw at you. This is effectively a memory error even though it does not look like one — or more specifically a multithreading issue that can be fixed by reducing the number of threads used. See here for some discussion:
I think the dada2 version was upgraded somewhere in there, so it could be a version issue, but it could also be coincidence.
Reduce the number of threads you are using and let us know if it works!