Have a look at this thread, it seems that offsite pages have been buggy for some: Cannot save Foundation Page
Either adding a ‘#’ in the field, or ensuring that a full website address is used seems to work for most. And it sounds like the beta of 7.3 is fixing it for some as well
(Edit: here’s the thread I was originally thinking of: BUG in RW 7.2.2 & 7.2.1) Again, just make sure the field isn’t blank