Ok, i figured it out how to do it, works nicely!
I just had to move the lookup record set above all the validation code otherwise it would not let me insert any values and always showed the error validation for the lookup.
Now it is working like a charm.
Just one question, when i enter some error message and select it to create the showif behavior it will generate the code nicely. I just wonder if you always need to select the correct class in the property inspector to create the same span tag with the same look like the other error messages which were created when inserting the form.
Also i found when selecting this class manually it does create the correct look but does not insert the id field, has this to be coded by hand as well?