Your issue could be because you're using double quotes. Try using single quotes instead around the URL to the FTP server. For example, here I'm using the free FTP service mentioned in this other SO Q&A titled: Is there a Public FTP server to test upload and download?.
$ identify 'ftp://demo:password@test.rebex.net/pub/example/WinFormClient.png'
ftp://demo:password@test.rebex.net/pub/example/WinFormClient.png=>WinFormClient.png PNG 800x700 800x700+0+0 8-bit sRGB 80000B 0.000u 0:00.000
I was able to find another site that allows you to upload files, https://dlptest.com/ftp-test/. In using this, I believe this replicates your issue:
$ identify 'ftp://dlpuser%40dlptest.com:e73jzTRTNqCN9PYAAjjn@ftp.dlptest.com/download with space.jpeg'
identify: no decode delegate for this image format `' @ error/constitute.c/ReadImage/512.
identify: no data returned `ftp://dlpuser%40dlptest.com:e73jzTRTNqCN9PYAAjjn@ftp.dlptest.com/download with space.jpeg' @ error/url.c/ReadURLImage/246.
This looks like a bug with Imagemagick's handling of URLs. This is the version that I'm using:
$ identify --version
Version: ImageMagick 7.0.8-0 Q16 x86_64 2018-06-13 https://www.imagemagick.org
Copyright: © 1999-2018 ImageMagick Studio LLC
License: https://www.imagemagick.org/script/license.php
Features: Cipher DPC HDRI Modules
Delegates (built-in): bzlib freetype jng jpeg ltdl lzma png tiff xml zlib
At any rate I see you've asked in the Imagemagick forums about this issue - Identify ftp and spaces.