After digging a little more, it looks like the default number format for the Dutch locale uses an “invalid” decimal number format.
Ignoring the escape characters, the number format matches what’s included in your initial post.
default.number.format=###.###.##0,00
This is a bug.
For now, we should set this to match the default number format in messages.properties. But we also need to investigate why this happens and how we’re supposed to get around this for all the countries (besides the US and UK) that use this number format pattern.
As a workaround, you should be able to edit the Dutch translations (message_nl.properties under the exploded WAR file within the Tomcat webapps directory) and override the default number format in that file.
For Tomcat 7, that file will likely be found here
/opt/tomcat/webapps/openboxes/WEB-INF/grails-app/i18n/messages_nl.properties
So then change the line
default.number.format=\#\#\#.\#\#\#.\#\#0,00
to
default.number.format=###,###,##0.00
Save the file and restart your application.
NOTE: Any new deployments will likely overwrite this file, so you’ll need to keep updating this until we provide a bug fix in an upcoming release.