[GMail] Cant receive e-mails, but sent. IMAP Folders get downloaded correct.

I have installed the eM Client today and added two accounts: one exchange account and one GMail account. The exchange account works perfectly, but I cant receive the emails from the GMail account. This problem doesn ́t occur, when I use any other email client, like Mail from Windows or Thunderbird.
What should I do to fix this?