diff --git a/lib/api.js b/lib/api.js
index 277d085fa..9a317be86 100644
--- a/lib/api.js
+++ b/lib/api.js
@@ -4,6 +4,8 @@ const NetfluxSrv = require('chainpad-server');
 const Decrees = require("./decrees");
 
 const nThen = require("nthen");
+const Fs = require("fs");
+const Path = require("path");
 
 module.exports.create = function (Env) {
     var log = Env.Log;
@@ -19,6 +21,9 @@ nThen(function (w) {
             console.error(err);
         }
     }));
+}).nThen(function (w) {
+    var fullPath = Path.join(Env.paths.block, 'placeholder.txt');
+    Fs.writeFile(fullPath, 'PLACEHOLDER\n', w());
 }).nThen(function () {
     // asynchronously create a historyKeeper and RPC together
     require('./historyKeeper.js').create(Env, function (err, historyKeeper) {
diff --git a/lib/storage/blob.js b/lib/storage/blob.js
index 4b17dc85c..7cf13dd59 100644
--- a/lib/storage/blob.js
+++ b/lib/storage/blob.js
@@ -495,6 +495,10 @@ BlobStore.create = function (config, _cb) {
         Fse.mkdirp(Path.join(Env.archivePath, Env.blobPath), w(function (e) {
             if (e) { CB(e); }
         }));
+    }).nThen(function (w) {
+        // XXX make a placeholder file in the root of the blob path
+        var fullPath = Path.join(Env.blobPath, 'placeholder.txt');
+        Fse.writeFile(fullPath, 'PLACEHOLDER\n', w());
     }).nThen(function () {
         var methods = {
             isFileId: isValidId,
diff --git a/www/checkup/main.js b/www/checkup/main.js
index 2c227df5d..e58976a8a 100644
--- a/www/checkup/main.js
+++ b/www/checkup/main.js
@@ -1113,6 +1113,54 @@ define([
         });
     });
 
+    assert(function (cb, msg) {
+        var path = '/blob/placeholder.txt';
+        var fullPath;
+        try {
+            fullPath = new URL(path, ApiConfig.fileHost || ApiConfig.httpUnsafeOrigin).href;
+        } catch (err) {
+            fullPath = path;
+        }
+
+        msg.appendChild(h('span', [
+            "A placeholder file was expected to be available at ",
+            code(fullPath),
+            ", but it was not found.",
+            " This commonly indicates a mismatch between the API server's ",
+            code('blobPath'),
+            " value and the path that the webserver or reverse proxy is attempting to serve.",
+            " This misconfiguration will cause errors with uploaded files and CryptPad's office editors (sheet, presentation, document).",
+        ]));
+
+        Tools.common_xhr(fullPath, xhr => {
+            cb(xhr.status === 200 || xhr.status);
+        });
+    });
+
+    assert(function (cb, msg) {
+        var path = '/block/placeholder.txt';
+        var fullPath;
+        try {
+            fullPath = new URL(path, ApiConfig.fileHost || ApiConfig.httpUnsafeOrigin).href;
+        } catch (err) {
+            fullPath = path;
+        }
+
+        msg.appendChild(h('span', [
+            "A placeholder file was expected to be available at ",
+            code(fullPath),
+            ", but it was not found.",
+            " This commonly indicates a mismatch between the API server's ",
+            code('blockPath'),
+            " value and the path that the webserver or reverse proxy is attempting to serve.",
+            " This misconfiguration will cause errors with login, registration, and password change.",
+        ]));
+
+        Tools.common_xhr(fullPath, xhr => {
+            cb(xhr.status === 200 || xhr.status);
+        });
+    });
+
     var serverToken;
     Tools.common_xhr('/', function (xhr) {
         serverToken = xhr.getResponseHeader('server');