diff --git a/src/cmd/cmd.cpp b/src/cmd/cmd.cpp
index 78be38da19d61..d55582588d64a 100644
--- a/src/cmd/cmd.cpp
+++ b/src/cmd/cmd.cpp
@@ -547,6 +547,15 @@ int main(int argc, char **argv)
 
     // Always try to load the user-provided exclude list if one is specified
     if (hasUserExcludeFile) {
+        if (!QFile::exists(options.exclude)) {
+            // A user-supplied --exclude path that can't be found is a
+            // configuration error, not something to silently ignore:
+            // reloadExcludeFiles() below drops missing files without
+            // failing, which previously made the whole sync run with
+            // no exclusions and no diagnostic (see nextcloud/desktop#4621).
+            qFatal("Exclude list file supplied via --exclude does not exist: %s", qUtf8Printable(options.exclude));
+            return EXIT_FAILURE;
+        }
         engine.excludedFiles().addExcludeFilePath(options.exclude);
     }
     // Load the system list if available, or if there's no user-provided list
diff --git a/src/csync/csync_exclude.cpp b/src/csync/csync_exclude.cpp
index 72e585aea7bc1..438fa9aac3851 100644
--- a/src/csync/csync_exclude.cpp
+++ b/src/csync/csync_exclude.cpp
@@ -346,6 +346,7 @@ bool ExcludedFiles::reloadExcludeFiles()
             const auto &excludeFile = *excludeFileIt;
             QFile file(excludeFile);
             if (!file.exists()) {
+                qWarning() << "Exclude list file does not exist, skipping:" << excludeFile;
                 excludeFileIt = excludeFiles.erase(excludeFileIt);
                 continue;
             }