Currently, "Related Records Filtering" on lookup fields works fine on Dynamics 365 (Online) but it doesn't work on Portal and occurs an error like "We're sorry, but something went wrong. Please try again, and if this persists, contact the website administrator."
And the error message says "????????? Web ???????" when the Japanese language is set.
It would be much better if the error message is appropriately displayed or "Related Records Filtering" works fine on Portal as well.