RapidWeaver 6.3.8 has been launched with many improvements.
There appears to be an issue with updating via RapidWeaver -> Check for Updates
so the recommended way to upgrade is to follow this link… http://realmacsoftware.com/rapidweaver/releasenotes/#6.3.8 and direct download the full version for installation.
The release notes are there as well 
Happy Weaving
Brad