Hello,
After DAVx⁵ has uploaded a task, the server replied with 204 No Content, which means that the task has been created correctly and there’s no further information about that.
A 204 response is terminated by the first empty line after the header fields because it cannot contain a message body.
In your case, the server has sent additional information (20 bytes) together with the status “there is no further information”, so this doesn’t make sense and can’t be processed.
It’s probably some misconfiguration of the server or the (reverse) proxy; please contact your admin for more information.