After updating to K2 2.7.0, we're having problems with the extra fields. On a category we've got an extrafield called: "Source", which only has to be filled out, if there is a source for the news item. But if we don't fill out the field, we cant published the item.

Any solution to this?

Check the setup for the extra field - it sounds like it's set to be 'required' which will prevent publishing of the item if it's not filled.

Exactly Kelsey is right, an item will not save if the field is required.