I realize this post is closed, but I wanted to add an update related to Magento ver. 2.3.2. I was running into this same error when uploading a csv that pulling a single image from an external url with no special characters or url related issues.

Magento 2 cannot import or download image from external resource

Imported resource (image) could not be downloaded from external resource due to timeout or access permissions

A quick check of the exception.log and I saw this:
[2019-11-20 23:59:35] main.CRITICAL: File directory 'pub/media/import' is not readable.

In my scenario, it was due to the pub/media/import directory missing completely. So, even if you are downloading a file from an external url, Magento uses the import directory to stage the file. Good to know.

There is far more detail at https://github.com/magento/magento2/issues/2666 if you are interested in seeing the life of this ticket.