Confirmed that the error is there.
Thanks for reporting. We'll look into fixing it.
Though just from looking at the exported atlas, it looks like it actually managed to pack the eff1 image into the atlas.
eff1
rotate: false
xy: 198, 11
size: 161, 160
orig: 161, 160
offset: 0, 0
index: -1
You can track the issue here: Export incorrectly shows "image [...] not found" warning. · #265 · EsotericSoftware/spine-editor