Reminder: You may need to edit this to fit the situation as this is only a guide.
When entering dates in SEIS, either use the Calendar or manually type in the dates. Do not let the field auto complete or autofill, doing so can cause errors or just not save the page.
Before trying this remove the dates and try saving. If you are able to save, logout of SEIS then back in and try re-entering the dates manually or by calendar.