This has been previously reported.
For More Information see: http://forums.filemaker.com/posts/6816363c73?start=1&stop=10#209950
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: https://www.dropbox.com/s/jt09b82i0xijbu3/FMP%20Bugs.zip
Thank you for the post.
I also went ahead and added your comments to the original report made to Testing and Development.
I had a similar issue and found the problem, at least what it was in my database:
Basically check that the next auto assigned ID in the portal ID record is higher than the record with the highest ID number.
The error is thrown because an ID is assigned that already exists, which causes a validation error if you have "Unique" validation selected.
My error was caused when I imported data which contained higher ID's than the field was defined to assign as the next ID.
The problem is that the error text does not directly convey that it is the validation check which fails. In my case it was the "Unique" validation that failed.
The error text misdirects your attention, by stating that the field requires a value (making you think the ID is empty, when it is not) and that the field is not available on the layout (which in this case is irrelevant as the ID field is autogenerated).
What the error SHOULD say is: "The portal field "xxx::yyy_ID" failed validation as it is not Unique."
That would direct your attention to the actual error.