I just upgraded to a 64 bit computer, and I've loaded X1 64bit.
When I open arrangements created previously I get a prompt that tells me RXP is missing.  Yet, oddly enough, I am able to add new instances of the RXP into the synth rack, and they work fine.  I just can't open the instances that are already there.  Any suggestions?

Rescan plugins? Maybe its looking for an earlier version of RXP or 32 bit version? Perhaps install 32 bit version of plugin to transfer settings to new 64 bit instance.