error 500 after entrie

Hi, first of all I really like EBR's looks and feutures alot.
I am working on my new website. There is a problem when someone makes an entry in EBR i get error 500 500 - JHtml: :bootstrap wordt niet ondersteund. Bestand niet gevonden. (it says translated: JHTML: : bootstrap not supported. File not found)

It is not only the white screen appearing but also the whole joomla site doesn't work any more until i restart my web browser. The EBR entrie does get posted
Page refreshing doesn't help.

I use joomla 2.5.8 with EBR 2.5-4

Hope you can help.
You can test here: newsite.goochelaar.eu/gastenboek.html

Thanks for taking a look at this error.
Jochem The netherlands

Please log in to see the response. Attention: You need a valid subscription to activate an account.