The problem lies in the fact that the plugin tries to deduce the branch to be created absolute path from the current trunk path.
The assumption is that the svn repository is using the following convention:
http://svnserver.com/projectName and within that you have a a trunk and a branches folder.
Our setup uses a completely different structure where the "trunk" folder is at the root and has a different name other than trunk. Within that "trunk" folder you have another 2 layers at least till you reach the project that has been checked out.
The "culprit" seems to be: svnmerge-plugin / src / main / java / jenkins / plugins / svnmerge / IntegratableProjectAction.java
A possible fix would be to also allow the user to define the full feature branch path from the root of the svn as an additional feature. That would allow us to create a feature branch anywhere within our svn structure since features branches can go in more than one "branch locations" (ie you have no way to automatically deduce the full branch path)
Its not critical but currently we cannot use this plugin due to this problem.
I have never developed for Jenkins but I am a Java Developer so I could give it a go myself as long as I am pointed in the right direction.