Hi Gus, first please make sure you update the plugin to 1.4.4 first.
After that see if there's still a blog in your site admin that does not look right (doesn't have the right, or even no, url) and needs deleting. Delete that blog (OFF COURSE MAKE SURE IT IS A WRONG BLOG THAT WE CREATED AND CAN BE DELETED, OTHERWISE DO NOT DELETE IT) and make sure to save the admin settings of the groupblog (this is important, even if you did not delete a blog). In most cases this should be enough, otherwise follow instructions below.
If you have access to your db, find the table wp_bp_groups and browse it to find the right id of the group with the error in question. Remember its id. Now browse the table called wp_bp_groups_groupmeta and find all 'meta keys' that match your group_id. Since you've deleted the blog that was linked to it, you need to now delete the meta keys we failed to delete for you. Select the meta keys which start with groupblog_ for the group_id in question and delete them.
You should now be able to go back to your group in BuddyPress and in your group blog admin you can select an existing, or create a new, blog.
If any of this does not make sense or you have additional questions let us know.
Sorry again for this terrible bug, but atm we can only rectify it by doing a manual delete.