slide doesn't work

The creation of the page breaks halfway. This is either a server problem or a conflict with an other plugin.

Add define('WP_DEBUG', true); in wp-config.php. This may give you a meaningfull errormessage from the server.

Try the same page while all other plugins are de-activated.

If you can not solve this, check Table IX-A3 ( Allow wppa+ debugging ) in the Photo Albums -> Settings admin page. This makes it possible for me to see more in your site to find the cause of the problem.

yes, now I see a lot of mistakes.
maybe it is the guestbook.

it says:

Fatal error: Call to undefined function esc_textarea() in /home/vhosting/j/vhost0039563/domains/degoudseclownschool.nl/htdocs/www/wp-content/plugins/wp-photo-album-plus/wppa-functions.php on line 1793

do you think that I need to update my version of WP?
I never did this before so I’m a little afraid I lose everything.
Will it solve maybe the problem?

I deactivited all plugins, but that doesn’t help.

Your WP must be version 3.1 or later, so update wp. You will not loose anything.