Game Will Not Start? SmartScreen, Launcher and Permission Fixes
Last updated: 10 July 2026
When MU Online will not start, the cause is almost always one of three things: Windows SmartScreen blocking the installer, the launcher failing to hand off to the game executable, or a silently corrupted download. Find the section below that matches what you actually see on screen — each fix ends with a short verification step so you know it worked before moving on.
This guide covers the RiftMu Season 21 client specifically, but the causes are the same on virtually any MU Online private server.
Case 1: Windows shows a blue popup when you run the installer
What you see: a blue full-width dialog saying Windows protected your PC, with no obvious way to continue. Your browser may also have warned you before the download finished.
Why it happens: Microsoft SmartScreen rates programs by download reputation, not by scanning them for anything harmful. Every time we release an updated RiftMu installer it is a brand-new file that has not yet been downloaded by enough people, so SmartScreen shows the generic warning until the new build earns its reputation. This is the same behaviour described on our download page, and it happens to every small publisher shipping a fresh build — it is not a malware verdict.
The fix:
- On the blue dialog, click the small More info link. The dialog expands to show the file name and publisher.
- Click Run anyway. This button only appears after More info — that is deliberate on Microsoft’s side.
- If your browser blocked the download itself (Edge and Chrome sometimes hold new files), open the downloads list, click the blocked entry and choose Keep before you even get to SmartScreen.
Only click Run anyway for an installer you downloaded yourself from the riftmu.com download page. If someone sent you the client as a zip on Discord, do not run it — get the official installer instead.
Verify it worked: the installer window opens and asks for an install location. Choose a simple path such as C:\Games\RiftMu rather than Program Files — it prevents the permission problems in Case 2.
Case 2: The launcher opens, but the game never starts
What you see: the launcher runs, maybe patches to 100%, you press Start — then nothing, or a brief flash, or a DLL error.
- Let patching finish completely. If the progress bar is still moving, the Start button may launch a half-updated client. Wait for the launcher to report it is up to date, then start. Verify: the launcher shows no pending files before you press Start.
- Run the launcher as administrator. Right-click it, Properties, Compatibility tab, tick Run this program as an administrator. The launcher must write patched files next to itself, and a standard user cannot do that inside Program Files. Verify: patching completes without Access denied lines and the game window appears.
- Fix missing DLL errors. Messages naming d3dx9_43.dll mean you need the Microsoft DirectX End-User Runtime (June 2010); messages naming msvcp140.dll or vcruntime140.dll mean you need the Visual C++ 2015–2022 Redistributable, both x86 and x64. Season 21 clients still use DirectX 9 libraries [Common on private servers], and a fresh Windows install does not include them. Verify: the exact DLL error no longer appears on launch.
- Check whether your antivirus quarantined the game executable. This is the classic launcher works, game does not case: the scanner removed main.exe while leaving the launcher alone. In Windows Security open Protection history, restore anything quarantined from your game folder, then add the game folder as an exclusion (Virus & threat protection, Manage settings, Exclusions, Add an exclusion, Folder). Do not turn real-time protection off globally — the folder exclusion is enough and the rest of your PC stays protected. Verify: the game executable is present in the folder and stays there after a relaunch.
Case 3: Nothing happens at all
What you see: you double-click the installer or launcher and literally nothing appears — no window, no error, no popup.
- Check Task Manager first. Press Ctrl+Shift+Esc and look for a stuck game or launcher process from a previous attempt. End it, then relaunch. A zombie process silently blocks new instances. Verify: no game processes are listed before you start again.
- Suspect a corrupt download. An installer that was cut off mid-download often does nothing at all when run. Delete it and download it again from the download page, ideally with a stable connection and no VPN. Verify: the new file’s size matches what the download page states, and the installer opens.
- Check free disk space. The client needs several gigabytes free for installation plus patching headroom. Windows sometimes fails a launch silently rather than warning you. Verify: your install drive has comfortable free space before retrying.
- Install pending Windows updates and restart. A half-installed Windows update can leave the system unable to start new DirectX applications until the restart completes. Verify: Windows Update reports you are up to date after the reboot, then try the launcher.
Symptom, cause and fix at a glance
| Symptom | Likely cause | Fix |
|---|---|---|
| Blue Windows protected your PC dialog | SmartScreen reputation on a new installer build | More info, then Run anyway (official installer only) |
| Browser blocks or discards the download | Same reputation system, applied at download time | Open downloads list, choose Keep, then run the file |
| Error naming msvcp140.dll or vcruntime140.dll | Visual C++ Redistributable not installed | Install VC++ 2015–2022, x86 and x64 |
| Error naming d3dx9_43.dll | Legacy DirectX 9 runtime missing | Install DirectX End-User Runtime (June 2010) |
| Launcher patches, Start does nothing | Antivirus quarantined the game executable | Restore from Protection history, exclude the game folder |
| Access denied during patching | Install inside Program Files without admin rights | Run launcher as administrator, or reinstall to C:\Games |
| Double-click does absolutely nothing | Stuck process or corrupt download | End old processes in Task Manager; re-download the installer |
Still stuck? When to contact support
If you have worked through the matching case above and verified each step, contact us through the contact page. Include your Windows version, your GPU model, the exact error text or a screenshot, and the list of steps you already tried — that usually turns a back-and-forth into a single reply. If the game starts but then crashes in-game, that is a different problem with its own guide: see the MU Online crashes guide.
Common mistakes
- Turning antivirus off entirely to run the installer. It leaves your whole system exposed and the quarantine usually re-triggers the moment protection comes back on. A folder exclusion is the correct, reversible fix.
- Treating the SmartScreen dialog as a malware verdict. It is a popularity check on a new file, not a scan result — but only bypass it for installers you fetched from the official download page yourself.
- Running the game from inside the zip archive or from a client copied off another server. Extract or install properly, and always use the current Season 21 client from /download.
- Pressing Start repeatedly while patching is unfinished, which stacks half-launched processes that then block each other. One launch attempt at a time, and clean up Task Manager between attempts.
- Skipping the verification steps. If you change five things and it works, you cannot tell your friend — or support — what the actual fix was, and you may have created new problems (like unnecessary compatibility flags) along the way.