Probably the widget doesn't handle the error. What if you use a regular edit box?
Hi Toon,
I think this message is not triggered by your validation rule, but by the domain model in which the attribute has a maximum length of 500. If you set it to unlimited, and check manually on the length with a validation rule, it might work correctly.
Or you might file a bug report, to make the widget handle this exception.
Are you returning false in the microflow on save? Did you try an error message instead of validation? Did you try microflow debugging?