Thank you Davide for answering this.
I set 755 permission on the TOMCAT_HOME/attachments folder.
It didn't help unfortunately even after restarting tomcat.
I have the same error log in Knowage report designer when deploying.
Downloading works fine and refreshing show a success message but the report template is not updated in the knowage server document.
I also spot an error in localhost_access_log :
"POST /knowage/sdk/DocumentsService HTTP/1.0" 500