I don't fully understand what you mean by not being able to 'load' folders from GMX. Assuming that this is an IMAP account, right click on the GMX inbox in your folder list in Thunderbird and select 'subscribe'. A window should open showing a list of folders, these should be the folders on your GMX account. At the top it should say 'select the folders to subscribe to'. Tick the folders that you want to appear in Thunderbird and press the 'subscribe' button. Then click OK at the bottom. At this stage I usually close Thunderbird and reopen it to refresh the display properly. You should now see the folders in your Thunderbird folder list. If there are many emails in the folders it may take a while for Thunderbird to download them all to your computer so be patient.