I had a similar problem splitting a repository ..
svndumpfilter: Invalid copy source path /dir/old_dir
What I did to get around the problem was to include the additional old directories that is was requesting, or that you know you moved. In my case I had moved 3 directories into another directory.
eg. Moved Folders A,B,C in to Folder D
cat project.dump | svndumpfilter include A B C D > new.dump
This seemed to solve my problem. I was able to separate Folder D from the rest of the Repo.
On the flip-side, when excluding D I did not get the error, I would guess because removing D didn’t require the links/history to A,B,C