When some users join the server, instead of downloading the resource pack, they get kicked with the error described in the screenshot.
"java.lang.IllegalArgumentException: Parameter 'directory' is not a directory"

I myself cannot reproduce this problem. It downloads and installs just fine, but several users have reported this issue.

They are able to join just fine if they don't enable the resource pack.

Initially I had it hosted on a Dropbox link as many others had. Then I thought there might be a problem with that... so I set up my own Apache server on the box I was working with. Again, always works fine for me and other players. A select but significant few cannot join the server with resource packs enabled.
I moved it to promethiaRP.com/resources.zip
and in resource-pack=, I used
"resource-pack=http\://promethiaRP.com/resources.zip"
The \: is proper to escape the colon.
Heading to the link in the browser after correcting the escape character works perfectly.
I had one user who was struggling with it completely reinstall her .minecraft
That causes it to completely crash instead. Wonderful

Could I please get some insight on this?