I used Rhythmbox for years to manage my music for my iPod classic. It stopped working with Lubuntu 18 (like lots of other stuff is different now). The iPod appears in Rhythmbox and data is readable but upon "synchronize" I get the error that the device can not be opened for write.
I will answer my own question since I think this might confuse a lot of people. It is not a problem of Lubuntu. I had to reset my iPod with iTunes to make it work again.
However, this is not easily understandable, since the iPod is shown in the file manager and in Rhythmbox and even on my OS X Mavericks iTunes does not show a message that it is corrupted.
Only when I installed iTunes on Windows 10 and tried to connect the iPod there was iTunes giving me the message that I had to reset the iPod. I first thought this might be related to the different operating systems Windows vs. OS X but it turned out that after I reset under Windows the iPod is now writable again under Lubuntu.