What I am doing:
Apply external properties to a folder in the trunk (both single file and folder external, externals are binary files)
Create a
I found a work-around.
Get the revision number for the branch you are going to integrate.
Update to revision (using TortoiseSVN), and put the branch revision in the indicated textbox.
check the box -> "Omit externals"
Now go and re-integrate the branch to the trunk and all works.
Note: I tried with multiple clients, SmartSVN and TortoiseSVN. They had the same error.