I created a fairly simple text based MA for a custom building object and had constructed an Inbound Synchronization Rule in FIM Service, but when I synched the rule into the Metaverse it failed with a sync-rule-inbound-flow-rules-invalid failure.
This was difficult to troubleshoot as there was no stack trace, which meant no other error messages with more detail were found. Cutting the inbound flows to just the building number anchor attribute didn’t help. I noticed in Metaverse Designer that it showed the building number now had an inbound flow even though the Synchronization Rule did not make it into the Metaverse, which I thought was strange.
I had deleted and recreated the MA and Synchronization Rule many times without correcting the error. Without jumping all the way back to a completely new install, I took another step back and recreated the object type and attributes that were going to be used.
It turned out, when I had first created the Object Type in FIM Sync, I had defined the anchor attribute as String (non-indexable). By deleting the Object Type and reconstructing it with String (indexable) attributes I was able to get the rule to sync into the Metaverse.
So the takeaway from this is that sync-rule-inbound-flow-rules-invalid may have nothing to do with the inbound flows defined in FIM Service at all. I suppose it makes sense that the anchor attribute should at least be indexable. But the error message sure could use some work.
 I did not create the attributes that way on purpose. We have so many object classes and attributes that we deal with I had written a script to populate the schema programatically and the script was creating all String attributes as non-indexable.