Power Query "Could not load file or assembly System.Memory, Version=4.0.1.2"

Hi jake_duct 
Based on the pattern you described, this looks much more like a Windows 11 / standard-user / security-policy / assembly-loading issue than a workbook or Power Query design problem.

Why that’s the best fit:

• Works as Administrator
• Works in Safe Mode
• Fails only in a normal standard-user session
• Only on Windows 11
• Error is a missing .NET assembly during Mashup evaluation: System.Memory, Version=4.0.1.2

That usually points to one of these:

• EDR / antivirus / application control interference
• Controlled Folder Access
• AppLocker / WDAC / policy restrictions
• Per-user cache or temp-folder permissions
• A Windows 11-specific compatibility regression
• A broken user profile or user-scoped Office/Mashup cache

What to try

1) Test with a brand-new Windows user profile

Create a fresh standard user and test the workbook there.

• If it works: the issue is likely profile-specific
• If it fails: the issue is more likely machine policy / security stack / OS build

2) Ask IT to check security controls

Since Safe Mode works, I’d focus first on:

• EDR / antivirus exclusions
• Controlled Folder Access
• AppLocker
• WDAC
• ASR rules
• restrictions on Office child processes or embedded runtime loads

Useful exclusions to test:

• EXCEL.EXE
• Office / Click-to-Run paths
• %LOCALAPPDATA%
• %TEMP%
• Power Query / Mashup cache locations

3) Compare Win10 vs Win11 policy state

Since the same build works on Windows 10, compare:

• cumulative updates
• Defender policies
• MDM / GPO settings
• EDR agent version and policy
• local security baseline differences

4) Clear user-side Office/Mashup cache

You already tried Office repair, which is good. But if the issue is profile-scoped, user-side cache cleanup may still matter.

5) Try a different Win11 patch level

If this started after a Windows update, test:

• a rollback, or
• a newer cumulative update

That can confirm whether this is a Windows 11 regression.

About System.Memory

System.Memory is often the symptom, not the real cause. Power Query is likely failing while loading something in the embedded runtime path, and the assembly load error is what bubbles up.

So the fix is usually not “manually install System.Memory” or add it to the GAC. The better fix is to remove whatever is blocking the Mashup engine from loading its dependencies under a standard user token.

Practical workaround

Until IT isolates it:

• run Excel elevated as a temporary workaround
• or use the Windows 10 machine for that workbook