diff --git a/clang-tools-extra/clangd/ParsedAST.cpp b/clang-tools-extra/clangd/ParsedAST.cpp
--- a/clang-tools-extra/clangd/ParsedAST.cpp
+++ b/clang-tools-extra/clangd/ParsedAST.cpp
@@ -289,8 +289,15 @@
       std::move(CI), PreamblePCH,
       llvm::MemoryBuffer::getMemBufferCopy(Inputs.Contents, Filename), VFS,
       ASTDiags);
-  if (!Clang)
+  if (!Clang) {
+    // The last diagnostic contains information about the reason of this
+    // failure.
+    std::vector<Diag> Diags(ASTDiags.take());
+    elog("Failed to prepare a compiler instance: {0}",
+         !Diags.empty() ? static_cast<DiagBase &>(Diags.back()).Message
+                        : "unknown error");
     return None;
+  }
 
   auto Action = std::make_unique<ClangdFrontendAction>();
   const FrontendInputFile &MainInput = Clang->getFrontendOpts().Inputs[0];
diff --git a/clang-tools-extra/clangd/Preamble.cpp b/clang-tools-extra/clangd/Preamble.cpp
--- a/clang-tools-extra/clangd/Preamble.cpp
+++ b/clang-tools-extra/clangd/Preamble.cpp
@@ -390,8 +390,8 @@
         SerializedDeclsCollector.takeMacros(), std::move(StatCache),
         SerializedDeclsCollector.takeCanonicalIncludes());
   } else {
-    elog("Could not build a preamble for file {0} version {1}", FileName,
-         Inputs.Version);
+    elog("Could not build a preamble for file {0} version {1}: {2}", FileName,
+         Inputs.Version, BuiltPreamble.getError().message());
     return nullptr;
   }
 }