Use https://github.com/jMonkeyEngine/sdk-update-center as the nightly plugin repo, good luck all!

jme3-core-updatecenters/src/com/jme3/gde/core/updatecenters/Bundle.properties

@@ -1,5 +1,5 @@
 #jMP update centers
-com_jme3_gde_core_update_center_nightly=http://updates.jmonkeyengine.org/nightly/3.1/plugins/updates.xml
+com_jme3_gde_core_update_center_nightly=https://raw.githubusercontent.com/jMonkeyEngine/sdk-update-center/nightly-xml/updates.xml
 com_jme3_gde_core_update_center_stable=http://updates.jmonkeyengine.org/stable/3.1/plugins/updates.xml
 com_jme3_jmp_contributions_update_center=http://updates.jmonkeyengine.org/contrib/3.1/updates.xml
 

jme3-core-updatecenters/src/com/jme3/gde/core/updatecenters/Installer.java

@@ -1,12 +1,42 @@
 /*
- * To change this template, choose Tools | Templates
- * and open the template in the editor.
+ * Copyright (c) 2003-2020 jMonkeyEngine
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ * * Redistributions of source code must retain the above copyright
+ *   notice, this list of conditions and the following disclaimer.
+ *
+ * * Redistributions in binary form must reproduce the above copyright
+ *   notice, this list of conditions and the following disclaimer in the
+ *   documentation and/or other materials provided with the distribution.
+ *
+ * * Neither the name of 'jMonkeyEngine' nor the names of its contributors
+ *   may be used to endorse or promote products derived from this software
+ *   without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
+ * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+ * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+ * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
+ * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+ * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 package com.jme3.gde.core.updatecenters;
 
 import org.openide.modules.ModuleInstall;
 import org.openide.util.NbPreferences;
 
+/**
+ * @author normenhansen
+ */
 public class Installer extends ModuleInstall {
 
     @Override

jme3-core-updatecenters/src/com/jme3/gde/core/updatecenters/layer.xml

@@ -11,7 +11,7 @@
                 <attr name="url" bundlevalue="com.jme3.gde.core.updatecenters.Bundle#com_jme3_gde_core_update_center_nightly"/>
                 <attr name="category" stringvalue="BETA"/>
             </file>
-            <file name="com_jme3_gde_core_update_center_stable.instance">
+            <!--<file name="com_jme3_gde_core_update_center_stable.instance">
                 <attr name="displayName" bundlevalue="com.jme3.gde.core.updatecenters.Bundle#Services/AutoupdateType/com_jme3_gde_core_update_center_stable.instance"/>
                 <attr name="enabled" boolvalue="true"/>
                 <attr name="instanceCreate" methodvalue="org.netbeans.modules.autoupdate.updateprovider.AutoupdateCatalogFactory.createUpdateProvider"/>
@@ -26,7 +26,7 @@
                 <attr name="instanceOf" stringvalue="org.netbeans.spi.autoupdate.UpdateProvider"/>
                 <attr name="url" bundlevalue="com.jme3.gde.core.updatecenters.Bundle#com_jme3_jmp_contributions_update_center"/>
                 <attr name="category" stringvalue="COMMUNITY"/>
-            </file>
+            </file>-->
         </folder>
     </folder>
 </filesystem>