Unfortunately the helpdesk wasn't able to send me anything at all. They just told me they were waiting for the engineering team to reply.
Here is something to try in the meantime, based on David's posting (whose experience I had unfortunately forgotten about). Your problem may also be cause by having used Greek characters from the Math Alphanumeric Symbols part of Unicode instead of the normal Greek part. It should not matter, but it did for him.
Doing a quick check I found one, a phi, shown in the graphic below:
Replacing that with a normal Greek phi might help. To check for others you need to do Edit > Find on all the lower case Greek chars in the Unicode range 1d6f0 to 1d720 that you might have used.
IMPORT SUCCESSFUL !!!
Thank you David and Tom! This is what I have done:
I have removed the following suspicious greek characters with latin "translation":
Χ -> "chi"
Φ -> "phi"
σ -> "sigma"
ν -> "nu"
ε -> "epsilon"
ψ -> "psi"
Π -> "PI" (capital pi)
I have only removed the letters from the main text NOT FROM THE FORMULAS inserted with the formula editor. This seems to be a really silly bug. I also did not remove the most common greek letters (α, β, Δ, θ, η and ρ).
There may also have been one single russian character which I have also found. However, I cannot tell which of the letters was the reason. I have already tried to submit the document so many times that I just removed everything in one go to save time.
I am so grateful since Apple Support still (after three weeks) did not even manage to reply. I'm now looking forward for the next stept to publish...
I hope when you have time you will use the communication channels you have established with them to tell them that their system is choking on characters from the Unicode Math Alphanumeric Symbols range (1D400 and up), how annoying this can be for authors of texts with equations, and that they really need to fix this. It does not make any sense for their system to require Unicode and then reject certain parts of it.
Now on the iBook Store!
Thanks everyone again for the help! I send an email to Apple Support with the description of the problem and they "will forward (my) comments to the appropriate team".
At least it took less than two days for review and luckily I did not have to change anything.