Yeah, thats the same exact situation that I had, I also had diamond in my storage. I removed the replicator with my wrench and laid it back and after that i have had no problems. Thanks though i appreciate it!
Did you insert the Crystal Memory into the lower slot of the Scanner GUI before scanning Iridium? It won't output the pattern directly into the Pattern Storage - you need to actually store the pattern on Crystal Memory disk, and then insert it into the Pattern Storage, then select the proper item at the Replicator.
If you already did so... then it's just a bug. It happened to me once, that after having patterns for Diamond and Iridium in the Pattern Storage, for some unexplained reason I could only select the Iridium at the Replicator, even though I had the Diamond in the Pattern Storage. Restarting the world helped.
