CSOM AddSubFolder no longer working in one of our SharePoint Online tenants > unkown error

[chr-sad]

Target SharePoint environment

SharePoint Online

What SharePoint development model, framework, SDK or API is this about?

CSOM

Developer environment

None

What browser(s) / client(s) have you tested

• 💥 Internet Explorer
• 💥 Microsoft Edge
• 💥 Google Chrome
• 💥 FireFox
• 💥 Safari
• mobile (iOS/iPadOS)
• mobile (Android)
• not applicable
• other (enter in the "Additional environment details" area below)

Additional environment details

CSOM with PowerShell 5.1.22000.2538

Describe the bug / error

As of today, it is no longer possible to create a folder in SharePoint Online using CSOM and the AddSubFolder() API in one of our tenants. Until yesterday (October 29, 2025), this worked without any problems. I tried it in this tenant in two different site collections, including a newly created list without any customization. When I run the exact same script on a second tenant in our company (also a newly created list without any customization), everything works fine.
As far as I can tell, no changes have been made to the configuration on our side. There are also no further error details.

Is anyone else experiencing this problem?

Steps to reproduce

...
$ParentList.Context.Load($ParentList.RootFolder)
$ParentList.RootFolder.AddSubFolder("test")
$ParentList.Context.Load($ParentList)
$ParentList.Context.ExecuteQuery()

unknown error in one tenant

Expected behavior

The folder "test" should be created without any error.

[Ashlesha-MSFT]

Hi @chr-sad,
Thank you for bringing this issue to our attention. We will look into it and get back to you shortly.
If you don’t receive a reply within two working days, please use this escalation form to escalate.

[chr-sad]

The problem still exists today (October 31, 2025) in our tenant. Hopefully it is only an issue with this specific tenant.

[Dee-san]

We are facing the same issue since 28th of october in our tenant - we didn't change a thing and have nothing to debug ("Unknown error")

[chr-sad]

@Dee-san That's not good—thanks for letting us know.
@Ashlesha-MSFT If this behavior reaches productive tenants, we will have a huge and critical problem, as all of our solutions will no longer work!

[Ashlesha-MSFT]

@chr-sad,
Based on the initial investigation, this appears to be a tenant-specific issue. I will tag the engineering team to escalate this issue so they can investigate the service behavior in your tenant.

To help us reproduce the issue accurately, could you please provide the following details:

• Tenant URL where the issue occurs
• Site collection and list/library name used in the script
• A test account or App Registration (if possible) with permissions to run the CSOM AddSubFolder() call

Once we have these details, provide all necessary information to the engineering team for further investigation.

[chr-sad]

@Ashlesha-MSFT thank you. We have already prepared your required informations. Is there a way to send this in a private way since i do not want to publish this sensitive data here on github?

Unfortunately, we now also have our first productive customer reporting this problem.

[Ashlesha-MSFT]

@chr-sad,
Thank you for preparing the requested information. To keep your tenant details private, please send them directly via email instead of posting them here.

You can share the details with me at v-ashlpawar@microsoft.com

[chr-sad]

@Ashlesha-MSFT
I have sent the requested information. We now have 3 productive customers with an outage. We now have three productive customers with one failure. We are currently trying a workaround with AddSubFolderUsingPath. Initial incomplete tests suggest that this could still work.

[chr-sad]

@Ashlesha-MSFT it seems, that in the newest Microsoft.SharePoint.Client.dll for SP Online the AddSubFolder API has changed.

It is no longer:
public void AddSubFolder(
string leafName
)

but requires a second parameter.