Hi Jose
Not sure if this helps, I have a DNN install with multiple portals, some have there own domain names and others are sub domains so main site www.mysite.com sub domain www.mysite.com/child
I have experienced a similar issue to yours
In the child portal if I select HyperLink --> Page it will select the page from the drop down and even if you look at the HTML all is OK. However when you save it "BANG!!!"
What was www.mydomain.com/child1/subpage.aspx becomes //subpage.aspx and results in an error because it's trying to find subpage.aspx on the main portal
The solution is to NOT pick the page from the drop down but to enter the full URL into the link
This issue is both for textual links and image links
Good luck
Dave