Sourceforge recently updated their SSL certificates, which triggered Subversion to re-validate and caused the error message "Error validating server certificate for 'https://...'" The new Sourceforge certificates are for hostname "*.svn.sourceforge.net", but it appears that Subversion does not recognize the * as a pattern to match all hostnames. Both Internet Explorer and Firefox recognize the certificate as valid when browsing to the directory manually. I hope I chose the right subcomponent. Please change it if it's not. It's possible the problem is in neon itself.
Original issue reported by siliconfiend