I've the following setup - 

• VS Code v1.88.1
• ZMF Explorer v2.0.5
• Zowe CLI v 7.16.6

The zowe.config.json has following - 

        "zmf": {
            "type": "zmf",
            "properties": {
                "basePath" : "zmfrestk",
				"host": "hosturl",
                "port": 8443,
                "protocol": "https",
                "rejectUnauthorized": true
            },
            "secure": [
                "user",
                "password"
            ]
        },
        "base": {
            "type": "base",
            "properties": {
                "host": "hosturl",
                "rejectUnauthorized": true
            },
            "secure": [
                "user",
                "password"
            ]
        }
    },
    "defaults": {
        "zosmf": "zosmf",
        "tso": "tso",
        "ssh": "ssh",
        "rse": "rse",
        "zmf": "zmf",
        "base": "base"
    },
    "autoStore": true

On "ZMF Explorer", in the tree view, upon clicking on 'Applications', I get this error - "Unable to establish a session with zmf". 

I the log file (), I see the following details - 

[2024/04/26 17:23:34.364] [DEBUG] [extensionHostProcess.js:154] CommonRest -createResource /logon
[2024/04/26 17:23:34.817] [ERROR] [task_queues:95] X5 [Error]: http(s) request error event called
    at oj.populateError (c:\\Users\\z181628\\.vscode\\extensions\\micro-focus-amc.zmfexplorer-2.0.5\\out\\extension.js:874:233)
    at ClientRequest.<anonymous> (c:\\Users\\z181628\\.vscode\\extensions\\micro-focus-amc.zmfexplorer-2.0.5\\out\\extension.js:850:19288)
    at ClientRequest.emit (node:events:529:35)
    at TLSSocket.socketErrorListener (node:_http_client:501:9)
    at TLSSocket.emit (node:events:517:28)
    at emitErrorNT (node:internal/streams/destroy:151:8)
    at emitErrorCloseNT (node:internal/streams/destroy:116:3)
    at processTicksAndRejections (node:internal/process/task_queues:82:21) {
  mDetails: {
    msg: 'http(s) request error event called',
    causeErrors: Error: write EPROTO 12813056:error:100000f7:SSL routines:OPENSSL_internal:WRONG_VERSION_NUMBER:..\\..\\third_party\\boringssl\\src\\ssl\\tls_record.cc:231:
    
        at WriteWrap.onWriteComplete [as oncomplete] (node:internal/stream_base_commons:94:16) {
      errno: -4046,
      code: 'EPROTO',
      syscall: 'write'
    },
    source: 'client',
    errorCode: undefined,
    protocol: 'https',
    port: 8443,
    host: 'hosturl',
    basePath: 'zmfrestk',
    httpStatus: undefined,
    errno: undefined,
    syscall: undefined,
    payload: undefined,
    headers: [ [Object] ],
    resource: '/logon',
    request: 'GET',
    additionalDetails: 'HTTP(S) client encountered an error. Request could not be initiated to host.\\n' +
      'Review connection details (host, port) and ensure correctness.\\n' +
      '\\n' +
      'Protocol:  https\\n' +
      'Host:      hosturl\\n' +
      'Port:      8443\\n' +
      'Base Path: zmfrestk\\n' +
      'Resource:  /logon\\n' +
      'Request:   GET\\n' +
      'Headers:   [{"Accept-Encoding":"br, deflate, gzip"}]\\n' +
      'Payload:   undefined'
  },
  mMessage: 'http(s) request error event called'
}

The ZMF Rest services are setup and accessing via web browser and also in zowe cli (using "ChangeMan ZMF Plug-in for Zowe CLI V2.0.5").

What could be wrong with ZMF Explorer on VSCode? Any help is appreciated.

Are you using a certificate as I see you are accessing with a protocol of HTTPS with rejectUnauthorized set to true? 
You could try changing to rejectUnauthorized to false as a test

No, I am not using any certificate. I tried with "rejectUnauthorized": false; but result is same.

Then you should probably be using HTTP instead of HTTPS and ensure that you are using the correct port number.and basePath etc

Thank you, David. I believe the issue is with our Changeman ZMF version, it's currently at version 8.2, patch level 5. I will re-try once it's updated to a newer version.