Bukkit

Hi, I am trying to disguise as a player that is not in my server, and I get the error "Error while fetching offline player UUID." My server is in online mode. The full log is:

[17:33:11 WARN]: [DisguiseCraft] Error while fetching offline player UUID
java.io.IOException: Server returned HTTP response code: 415 for URL: https://api.mojang.com/profiles/page/1
at sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1894) ~[?:1.8.0_171]
at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1492) ~[?:1.8.0_171]
at sun.net.www.protocol.https.HttpsURLConnectionImpl.getInputStream(HttpsURLConnectionImpl.java:263) ~[?:1.8.0_171]
at pgDev.bukkit.DisguiseCraft.mojangauth.UUIDFetcher.call(UUIDFetcher.java:42) ~[DisguiseCraft.jar:?]
at pgDev.bukkit.DisguiseCraft.mojangauth.ProfileCache.cache(ProfileCache.java:59) [DisguiseCraft.jar:?]
at pgDev.bukkit.DisguiseCraft.listeners.DCCommandListener.playerDisguiseCommand(DCCommandListener.java:1702) [DisguiseCraft.jar:?]
at pgDev.bukkit.DisguiseCraft.listeners.DCCommandListener$1.run(DCCommandListener.java:1688) [DisguiseCraft.jar:?]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [?:1.8.0_171]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [?:1.8.0_171]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_171]

Please help, because it would be very cool if I could disguise as any player.