I don’t have an answer to your question but I wonder if a set of checkboxes might work for you.
I am sorry to say that it wouldn’t, this is only a sample of the list, it has over 30 options so the check boxes will not be practical. Thanks for the idea. I have various standard select boxes that work fine, just the multiple one does not.
It will only go through if there are no options selected.
Also there are some special characters in the option values. I wondr if that could be a problem.
EDIT: removing the special chars from the values did the trick.