No, it's not only you.
I can't change my seats either. I tried changing browser but that didn't help. Everything acts and looks normal all the way until after I save the changes. Then I get a lot of error messages and also an e-mail with this very interesting message:

The following warning(s) occured
At least one of your specific seat requests could not be processed. (4631) At least one of your specific seat requests could not be processed. (4631) At least one of your specific seat requests could not be processed. (4631) At least one of your specific seat requests could not be processed. (4631) At least one of your specific seat requests could not be processed. (4631) Your reservation is confirmed, but your special request could not be processed for these flights. Please contact us for further information. (4609) (4609)

Very useful!! Btw I am only trying to change my seat on one of my four segments.
And I get this everytime I try.