The guestbook entries are stored in your MySQL database. Upgrading will not remove them. There is no need to use the install.php file if you are upgrading from 2.3.1. The only two template files that have possibly changed are entry.php and email.php.
I've never used the poll so couldn't say for certain. It will probably be one of the template files. Also I got my code slightly wrong. It should be assuming that is that Tree is the name you gave to the frame containing the menu.
Did you set $LANG[FormPreview] in the language file to say
Se det før det gemmes
? I think the problem may lie in the ø character. You are using the HTML entity which is correct but possibly it is getting sent as the actual character and so is causing the error.