Sounds like possibly one of your files has been altered or is missing. It would usually say if one was missing though. Have you altered lib/gb.class.php, templates/form.php or form/url.php ?
No. As you have not edited any of the templates simply replace all your current files with the 2.3.3 files EXCEPT your config.inc.php file. You can keep the guestbook in Danish simply edit the danish.php file found in the lang folder to change the preview button text.
The script uses the text on the button to know what to do but the $oslash; is sent as the actual character whilst the script expects ø and so thinks nothing is to be done and reshows the form.
Go into phpMyAdmin and if needed to the guestbooks database. Click book_config in the left pane. You should get a list up in the right pane of the fields for book_config. Look for notify_mes and click the edit icon to he right of it (read the little pop up you get when you mouse over the images). You need to change the type from varchar to text and then you will have unlimited space.