diff --git a/app/client/models/AdminChecks.ts b/app/client/models/AdminChecks.ts
index 40baf812..96a6f8d7 100644
--- a/app/client/models/AdminChecks.ts
+++ b/app/client/models/AdminChecks.ts
@@ -162,6 +162,16 @@ It is good practice not to run Grist as the root user.
   'reachable': {
     info: `
 The main page of Grist should be available.
+`
+  },
+
+  'websockets': {
+    info: `
+Websocket connections need HTTP 1.1 and the ability to pass a few
+extra headers in order to work. Sometimes a reverse proxy can
+interfere with these requirements. See <a
+href="https://support.getgrist.com/self-managed/#how-do-i-run-grist-on-a-server">this
+documentation</a> for more explanation.
 `
   },
 };
diff --git a/app/common/BootProbe.ts b/app/common/BootProbe.ts
index 753af1c4..699d3515 100644
--- a/app/common/BootProbe.ts
+++ b/app/common/BootProbe.ts
@@ -6,7 +6,8 @@ export type BootProbeIds =
     'reachable' |
     'host-header' |
     'sandboxing' |
-    'system-user'
+    'system-user' |
+    'websockets'
 ;
 
 export interface BootProbeResult {
diff --git a/app/server/lib/BootProbes.ts b/app/server/lib/BootProbes.ts
index 6c21c8a0..3dd02bf9 100644
--- a/app/server/lib/BootProbes.ts
+++ b/app/server/lib/BootProbes.ts
@@ -4,6 +4,7 @@ import { removeTrailingSlash } from 'app/common/gutil';
 import { expressWrap, jsonErrorHandler } from 'app/server/lib/expressWrap';
 import { GristServer } from 'app/server/lib/GristServer';
 import * as express from 'express';
+import WS from 'ws';
 import fetch from 'node-fetch';
 
 /**
@@ -58,6 +59,7 @@ export class BootProbes {
     this._probes.push(_bootProbe);
     this._probes.push(_hostHeaderProbe);
     this._probes.push(_sandboxingProbe);
+    this._probes.push(_webSocketsProbe);
     this._probeById = new Map(this._probes.map(p => [p.id, p]));
   }
 }
@@ -104,6 +106,37 @@ const _homeUrlReachableProbe: Probe = {
   }
 };
 
+const _webSocketsProbe: Probe = {
+  id: 'websockets',
+  name: 'Can we open a websocket with the server',
+  apply: async (server, req) => {
+    return new Promise((resolve) => {
+      const url = new URL(server.getHomeUrl(req));
+      url.protocol = (url.protocol === 'https:') ? 'wss:' : 'ws:';
+      const ws = new WS.WebSocket(url.href);
+      const details: Record<string, any> = {
+        url,
+      };
+      ws.on('open', () => {
+        ws.send('Just nod if you can hear me.');
+        resolve({
+          success: true,
+          details,
+        });
+        ws.close();
+      });
+      ws.on('error', (ev) => {
+        details.error = ev.message;
+        resolve({
+          success: false,
+          details,
+        });
+        ws.close();
+      });
+    });
+  }
+};
+
 const _statusCheckProbe: Probe = {
   id: 'health-check',
   name: 'Is an internal health check passing',
diff --git a/package.json b/package.json
index 44b167b0..73a8b7a9 100644
--- a/package.json
+++ b/package.json
@@ -7,7 +7,8 @@
   "repository": "git://github.com/gristlabs/grist-core.git",
   "scripts": {
     "start": "sandbox/watch.sh",
-    "start:debug": "NODE_INSPECT=1 sandbox/watch.sh",
+    "start:debug": "NODE_INSPECT=--inspect sandbox/watch.sh",
+    "start:debug-brk": "NODE_INSPECT=--inspect-brk sandbox/watch.sh",
     "install:python": "buildtools/prepare_python.sh",
     "install:python2": "buildtools/prepare_python2.sh",
     "install:python3": "buildtools/prepare_python3.sh",
diff --git a/sandbox/watch.sh b/sandbox/watch.sh
index 6cb7a750..531c97f6 100755
--- a/sandbox/watch.sh
+++ b/sandbox/watch.sh
@@ -19,6 +19,6 @@ tsc --build -w --preserveWatchOutput $PROJECT &
 css_files="app/client/**/*.css"
 chokidar "${css_files}" -c "bash -O globstar -c 'cat ${css_files} > static/bundle.css'" &
 webpack --config $WEBPACK_CONFIG --mode development --watch &
-NODE_PATH=_build:_build/stubs:_build/ext nodemon ${NODE_INSPECT:+--inspect} --delay 1 -w _build/app/server -w _build/app/common _build/stubs/app/server/server.js &
+NODE_PATH=_build:_build/stubs:_build/ext nodemon ${NODE_INSPECT} --delay 1 -w _build/app/server -w _build/app/common _build/stubs/app/server/server.js &
 
 wait