It is not possible to change the world, in my account dromogabri, (a***@gmail.com), I am stuck in the world 10 Spain, I can not return to Veterans 6. This is common to all accounts, you can not change the world . Similarly, to enter the account, any of them, it is necessary to reload many times the page, even exit, restart and return, the page is finally loaded, but you can not change the world either. Finally, in Veterans 6, in none of the three accounts with which I play, I mark the missions. Despite having destroyed several enemy bases, it does not mark me having done it, I can not collect the items that it rewards because it does not count on it, although it does mark it in the player information and score in the alliance. Sorry for the translation, I enter the comment in English and translate it into Castallian automatically.
EDIT: Email address removed GD