Cannot start listing using WebDAV server: "An image upload operation returned, but didn’t provide an image URL"

I have just set up a WebDAV Image Server, and told GS to use it to upload images. When I set up the WebDAV server in the Prefs > Image Server tab, and click the Test Settings… button, everything works fine: GS successfully uploads its FTPTestImage.jpg file to my server. So I’m confident my Image Server settings are correct.

But when I click Start Listing in Launch Control, it says it’s Uploading WebDAV image to [server name], and then pops up an error dialog saying, “An image upload operation returned, but didn’t provide an image URL”. It does this 8 times, once for each image in the listing. Launch Control then shows a green checkmark and Completed Successfully at the top, but 0 of 1 uploaded at the bottom, and the listing is not started.

What is the problem here, and how can I fix it? (I’m using the latest GS v9.9.5 (1736)).

I’m trying to make this WebDAV Image Server work because I want to have decent-sized images in my listings, and GS doesn’t seem to support that except with my own WebDAV server. When GS uploads to eBay’s EPS Image Server, it shrinks my images to 800 pixels (half of eBay’s recommended 1600 pixels). And while GS’s free picture service supports 1600 pixels, it does not support them for Good 'Til Cancelled listings, and all my listings are Buy It Now (not auctions), so they have to be GTC. I’m unwilling to pay $15/month for GS Pro just to be able to use the full-sized images eBay recommends.

How can I solve this problem?