Fix ERROR_TOO_MANY_MUXWAITERS 0x00000098 on Windows

Quick Answer

Restart the machine. If that doesn't work, open Task Manager, kill all processes named CONIME.EXE or WINOLDAP, then run sc stop mrxsmb10 in an admin command prompt.

Why This Happens

This is a legacy error from the Windows NT era. DosMuxSemWait is a 16-bit API that lets a thread wait on multiple semaphores at once. The limit is 64 semaphores per mutex — and once you hit it, nothing new gets through. The culprit here is almost always an old 16-bit or DOS application that leaked semaphores, or the mrxsmb10 (SMB 1.0) driver misbehaving. I've seen this on Windows 10 1809 and Windows 11 22H2 mostly after a printer driver install gone wrong, or when someone runs a legacy accounting app from the 90s.

Fix Steps (in order)

Restart the machine. I know — it's cliché. But semaphore leaks often clear on reboot because they're tied to session handles. Do this before anything else.
Kill old processes. Open Task Manager (Ctrl+Shift+Esc). Sort by process name. Look for CONIME.EXE or WINOLDAP. These are the 16-bit Windows compatibility layers. End both tasks. If the error goes away, that app is the problem.
Disable SMB 1.0. Open an admin command prompt and type:
```
sc stop mrxsmb10
```
Then
```
sc config mrxsmb10 start= disabled
```
This stops the SMB 1.0 driver from creating semaphore locks. Don't bother with SMB 2/3 — they don't use DosMuxSemWait.
Check for leaked handles. Download Process Explorer from Sysinternals. Press Ctrl+F, search for DosMuxSemWait. You'll see which process holds the most semaphore handles. Kill it from Process Explorer. Nine times out of ten it's a RPCSS.EXE or SVCHOST.EXE hosting the MupSvc service.

Alternative Fixes If Nothing Worked

Run the System File Checker: sfc /scannow. This rarely fixes semaphore issues, but I've seen it fix a corrupt mrxsmb.sys that was leaking handles. Takes 10 minutes, no harm.
Update or roll back the printer driver. If this error happened after a printer install, the driver is probably using a 16-bit port monitor. Go to Print Management (type printmanagement.msc), find the printer, remove it, and reinstall using a manufacturer's latest driver (not the Windows Update version).
Boot into Safe Mode with Networking. If the error's gone there, it's a third-party service. Use msconfig to disable non-Microsoft services one by one until the error stops.

Prevention Tip

Don't run 16-bit applications on modern Windows. If you must, use a virtual machine with Windows XP or DOSBox. The NTVDM subsystem is fragile and often triggers this error when left running for days. Also, keep SMB 1.0 disabled — it's been deprecated since Windows 10 version 1709. If you need file sharing, use SMB 3.0 instead. No reasonable app needs DosMuxSemWait in 2024.