I'm having trouble with org-attach-attach if my current buffer is visiting a filepath starting with "~/". This trivial patch fixes the problem for me by running (expand-file-name) on the file before attaching. I always use the 'lns method, so I don't know whether it might be better to expand-file-name before running any of the methods.
Alternatively, it might actually be better to use a *relative* file path where possible (e.g. to ensure that project directories are fully portable), but I'm not sure how best to do that.
This should be a tinychange, but also I have signed FSF paperwork.
0001-org-attach-attach-expand-file-names-before-linking.patch