I tested release 2.4.0 on a fresh W11 Home and got the following error.

I could not reproduce, even after removing the Roaming folder.

It looks like a race condition where we didn't wait/retry enough before deciding that we wouldn't be able to contact aria2. Actually, clicking OK did not prevent kiwix-serve from working at all and I could retrieve the catalog and even download a ZIM file from it so clearly aria2 did start.