Marco,
I have a severe problem with RAW-files and IPTC Data in XMP sidecar files. In order to keep the color tags and other metadata, I have set PM to always add XMP. I then open the files in Bridge CS4 where I need to further adjust captions etc. and postprocess the files.
When I try to change the caption, I get the follwong error message: "There was an error writing metadata to "[filename].cr2".
After this message which I have to confirm for every single file I had selected, the XMP seems to be reset because all post-processing alterations in ACR are gone.
I have the following settings for IPTC (using V 4.61):
Never embed IPTC4XMP on RAW files if you're using Adobe's products. Each of Adobe's imaging products (Photoshop, Bridge, and Photoshop Lightroom) have a bug where they prefer embedded XMP over sidecar XMP though they refuse to update IPTC4XMP inside RAW files. This causes their software to get out of sync with their own settings. The solution is to uncheck the "Embed IPTC4XMP" checkbox. Embedding IPTC is OK, but is unnecessary. Also, you should change the read order of the data to: (1) Sidecar XMP (2) Embedded IPTC (3) Embedded XMP
To repair any files which you are having problems with you can use the "Revert TIFF-based RAW to original" command on them, backing up your metadata into XMP sidecar files.
This should solve your problems.
-Kirk
BTW, what software was giving you the error? Photo Mechanic or something else?