Adriano,
First of all, the CRED developer sent a solution to someone to fix his problem according to this ticket – "CRED says "Post delete failed" even though it's not". So maybe there is a known solution to this problem.
Secondly, here are the credentials for the problem page:
Site: http://dev.soupenzo.nl/soupenzo-app-spiegelstraat/
Password: spiegel54
The area of interest is in the messages section. I will create a few test messages that you can delete to see the problem. Click on the 'X' to the right of the message.
Also, I was looking at the code and noticed that the delete anchor tag contains a target like this: target="_cred_cred_836_1_10_iframe". It seems that the code is somehow looking for an iframe when I am not using an iframe but rather the textarea for the post_content.
Sincerely,
Jason Thane Jeffers