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AE exports are blocked by a message stating IPTC metadata could not be included

Some users are being affected by blocking dialogs that state IPTC metadata could not be included in one or more versions.

There are two potential causes of this:

1. You have RAW+JPEG pairs in your Aperture library and the pair is set to use the JPEG as the master.

Solution: In Aperture search out your RAW+JPEG pairs (smart albums are the way to go for this) and instruct Aperture to use the RAW as the master.

2. There are simply jpegs that are "incompatible" with Aperture and metadata.

I’ve been investigating the dialog where IPTC metadata could not be included for some jpegs. It is not a permissions error nor is it a problem with the aperture library. It seems there are some cases where Aperture just can’t write out the IPTC metadata to jpegs for some crazy reason.
In the case I’ve examined even using Aperture’s ability to write IPTC metadata to the original fails. It is impossible to tell before trying the export if a particular jpeg is “incompatible”. I would create metadata sidecars for all the jpegs and it would avoid the exporting problem. However, lightroom does not use the sidecar files for jpegs. Catch 22!  
A workaround to this using exiftool is documented in The Mystery of Aperture Not Able to Embed Metadata Into Some JPEGs 

Unfortunately it is impossible to suppress these blocking messages. They are actually generated by Aperture. I certainly would suppress them until the end of the export if I could. Sorry. :(