TLDR: it’s this Julia issue (also reported here), caused by a problem with the OpenBLAS package where an experimental feature threading feature was accidentally enabled in the package used for the Julia release. Seems like it will require a rapid Julia 1.7.1 bugfix release.
Workaround is to call BLAS.set_num_threads(1) for now 
, or downgrade to Julia 1.6.3 until a fix is released.