Thanks for the report.  Based on those versions, the desktop Chrome M63 will try to load a version of the DevTools frontend that corresponds to M66, which the M63 version may not support.  This compatibility issue might be resolved if you use remote debugging with a desktop version M66.  Could you please try this using desktop Chrome Canary?

Also, if this fails on Canary, could you please check if there are console errors on the desktop DevTools front-end, by opening DevTools on DevTools?

Tried with this version of chrome on desktop i.e chrome-unstable: 65.0.3322.3 (Official Build) dev (64-bit)

Still the same issue. The exact error is : 404 Not Found The resource could not be found.

Regarding getting console errors could you give me more elaborate instructions?

I have this bug on Linux Version 64.0.3282.119 (Official Build) (64-bit)

I get these errors on the console for the devtools:
Uncaught TypeError: InspectorFrontendHost.setOpenNewWindowForPopups is not a function
    at Main.BackendSettingsSync._updateAutoAttach (inspector.js:8549)
    at new Main.BackendSettingsSync (inspector.js:8547)
    at Main.Main._createAppUI (inspector.js:8491)
    at Main.Main._gotPreferences (inspector.js:8480)
    at DevToolsAPIImpl.embedderMessageAck (devtools_compatibility.js:33)
    at <anonymous>:1:13

Uncaught TypeError: Cannot read property 'handleShortcut' of undefined
    at Main.Main._postDocumentKeyDown (inspector.js:8512)
6inspector.js:8512 Uncaught TypeError: Cannot read property 'handleShortcut' of undefined
    at Main.Main._postDocumentKeyDown (inspector.js:8512)


I tried with Version 65.0.3325.18 (Official Build) dev (64-bit) and inspecting my device seems to work fine. 
Canary on Windows also works fine.

Sorry for not responding earlier to c#3, the info in c#4 is helpful.

dgozman@, do you know if we landed something to fix compatibility in M65, but not merge it to M64?

We do not support forward-compatibility at all. Inspecting fresh build from mobile device might not work on stable chrome.

Perhaps, we should add a warning to chrome://inspect for that. Eugene, mind adding a warning when remote version is newer than host version?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d0a01560e2845547a92a05c60e0feb268d5e672f

commit d0a01560e2845547a92a05c60e0feb268d5e672f
Author: Dmitry Gozman <dgozman@chromium.org>
Date: Thu Feb 22 03:20:55 2018

[DevTools] Expose 'inspect fallback' on chrome://inspect

This is only shown if the remote browser is newer than host browser.
Since we don't support forward compatibility, the best we can do
is to fallback to local frontend hoping that helps a bit.

Bug:  803988 
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I2ef248dfd5ddae9c5ac9c335a5172e08a8c5b8e2
Reviewed-on: https://chromium-review.googlesource.com/929352
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Eugene Ostroukhov <eostroukhov@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#538329}
[modify] https://crrev.com/d0a01560e2845547a92a05c60e0feb268d5e672f/chrome/browser/devtools/devtools_window.cc
[modify] https://crrev.com/d0a01560e2845547a92a05c60e0feb268d5e672f/chrome/browser/devtools/devtools_window.h
[modify] https://crrev.com/d0a01560e2845547a92a05c60e0feb268d5e672f/chrome/browser/resources/inspect/inspect.css
[modify] https://crrev.com/d0a01560e2845547a92a05c60e0feb268d5e672f/chrome/browser/resources/inspect/inspect.js
[modify] https://crrev.com/d0a01560e2845547a92a05c60e0feb268d5e672f/chrome/browser/ui/webui/inspect_ui.cc
[modify] https://crrev.com/d0a01560e2845547a92a05c60e0feb268d5e672f/chrome/browser/ui/webui/inspect_ui.h

Marking as fixed, since we do not plan to fix underlying issue, and UI improvement has landed.

 Issue 815000  has been merged into this issue.