In trying to download an SDF representation of ZINC61878141, I found the downloaded file empty.
Looks like it is a duplicate of ZINC18177104. The database got confused because somehow the second copy sneaked in. The workaround for now is to use 18177104 instead.
Sorry for any inconvenience caused.
[We are working on a new version of ZINC that will, we hope, eliminate this kind of problem. But it won't be ready for testing until at least the end of the summer.]