One of the more frustrating parts of FIM 2010 is troubleshooting RCDC failures. Since most of the edits are done in XML they can be very prone to error. On more than one occasion, I have spent many hours running IISRESET and staring at blocks of XML.
A very good way to get back some time is to enable FIM portal tracing. This will allow you to get exactly to the error and give you an idea of what needs to happen in order to resolve the issue.
You can find instructions on how to enable this at http://social.technet.microsoft.com/wiki/contents/articles/how-to-configure-fim-portal-tracing.aspx.
In short, you just need to edit the SharePoint web.config and add the system.diagnostics node (followed by an IISRESET of course).
Below is an example of the svc trace produced from the logging. As you can see, knowing exactly which line and position the error occurred is very useful and helps to save a ton of time. Instead of wading through 2000+ lines of XML, I go straight to line 2039 at character position 78. Voila, root cause found we can now fix it and move on.
Just remember to reset the configuration file because a trace can consume a tremendous amount of disk space in a short time and it also has a performance cost associated with it as well.
Happy troubleshooting!