Thanks for your reply. After clearing the cache and reset file/folder-permissions the error in the campaign-builder is gone so I can test it.
Unfortunately, I continue to have problems sending text messages. I have created my own post about this at Problem sending text messages/SMS.
Attached is the information about the configuration:
But sending a text message/sms via a campaign now worked. Only sending a segment message didn’t work. But with that I can continue testing and working.