first of all a message to squall: please do not post unrelated URLs in this forum - the URL will be removed.
Now to the new validation message - there is a tick box option at the bottom of the priority section saying 'It is not intended to file a (further) priority claim' - the ticking of this option will remove the mentioned validation message. The reason for this message is due to the fact that with the EPC2000 you can file a priority claim until 16 month of the priority date and thus the EPO will not process the filing until this period is over or if the applicant has indicated that there is no intention to file a further claim. The declaration does not mean, however, that a later priority claim will be admissible or not - that will then have to be decided on the individual case.
Does this explain it for you?