Tested from multiple clients (Linux and Windows) and multiple ISPs. I'm guessing it is a server issue.
--2018-02-17 17:24:54-- (try:17) https://xlights.org/downloads/xLights64_2018_05.exe
Connecting to xlights.org (xlights.org)|67.222.33.247|:443... connected.
HTTP request sent, awaiting response... 206 Partial Content
Retrying.
Note: 32-bit works fine, so it appears to be limited to the 64-bit file.