--- a/masterfs/src/org/netbeans/modules/masterfs/filebasedfs/FileBasedURLMapper.java	
+++ a/masterfs/src/org/netbeans/modules/masterfs/filebasedfs/FileBasedURLMapper.java	
@@ -126,7 +126,7 @@ 
         FileObject retVal = null;
         File file;
         try {
-            file = FileUtil.normalizeFile(new File(url.toURI()));
+            file = FileUtil.normalizeFile(uri2File(url.toURI()));
         } catch (URISyntaxException e) {
             LOG.log(Level.INFO, "URL=" + url, e); // NOI18N
             return null;
@@ -138,6 +138,14 @@ 
         retVal = FileBasedFileSystem.getFileObject(file, FileObjectFactory.Caller.ToFileObject);
         return new FileObject[]{retVal};
     }
+    
+    static File uri2File(URI uri) { // #207060: UNC; candidate for API (#46813)
+        if (uri.getHost() == null) {
+            return new File(uri);
+        } else {
+            return new File("\\\\" + uri.getHost() + uri.getPath().replace('/', '\\'));
+        }
+    }
 
     private static URL fileToURL(final File file, final FileObject fo) throws MalformedURLException {
         URL retVal = toURI(file, fo.isFolder()).toURL();
--- a/masterfs/test/unit/src/org/netbeans/modules/masterfs/filebasedfs/FileBasedURLMapperTest.java	
+++ a/masterfs/test/unit/src/org/netbeans/modules/masterfs/filebasedfs/FileBasedURLMapperTest.java	
@@ -50,6 +50,7 @@ 
 import org.openide.filesystems.URLMapper;
 import org.openide.filesystems.FileUtil;
 import java.net.URL;
+import org.openide.util.Utilities;
 
 /**
  * @author Radek Matous
@@ -91,4 +92,14 @@ 
         assertEquals("192.168.0.201", uri.getHost());
         assertEquals("/data/services/web/com_resource/", uri.getPath());
     }
+    
+    public void testReverseUNCPath() throws Exception {
+        if (!Utilities.isWindows()) {
+            return;
+        }
+        assertEquals("C:\\some\\random path", FileBasedURLMapper.uri2File(new URI("file:/C:/some/random%20path/")).getAbsolutePath());
+        assertEquals("C:\\some\\random path", FileBasedURLMapper.uri2File(new URI("file:///C:/some/random%20path/")).getAbsolutePath());
+        assertEquals("\\\\server\\share\\some\\random path", FileBasedURLMapper.uri2File(new URI("file://server/share/some/random%20path/")).getAbsolutePath());
+    }
+    
 }
--- a/o.n.bootstrap/src/org/netbeans/JarClassLoader.java	
+++ a/o.n.bootstrap/src/org/netbeans/JarClassLoader.java	
@@ -946,7 +946,7 @@ 
             AGAIN: for (;;) try {
                 final URI uri = new URI(filePath);
                 if (uri.getScheme().equals("file")) {
-                    jar = new File(uri).getPath();
+                    jar = uri2File(uri).getPath();
                 } else {
                     jar = null;
                 }
@@ -981,6 +981,14 @@ 
             return new NbJarURLConnection (u, _src, _name, loader);
         }
 
+        private static File uri2File(URI uri) { // #207060: UNC; candidate for API (#46813)
+            if (uri.getHost() == null) {
+                return new File(uri);
+            } else {
+                return new File("\\\\" + uri.getHost() + uri.getPath().replace('/', '\\'));
+            }
+        }
+
         @Override
         protected void parseURL(URL u, String spec, int start, int limit) {
             if (spec.startsWith("/")) {