Accessing External Sharepoint Site

The error:

"Your account has been locked. Contact your support person to unlock it..."

is typically generated by Microsoft Entra ID (Azure AD) during authentication, not by SharePoint itself. The fact that it appears on the external company's branded login page suggests that authentication is reaching their Microsoft 365 tenant and then being rejected.

A few possibilities:

1. The external company no longer allows anonymous or broad external access

Many organizations have tightened SharePoint access so that users from other organizations must authenticate through an Entra B2B account. Your enterprise data contains references to Microsoft's migration from older SharePoint external-sharing methods toward Entra B2B guest accounts. External access that used to work may now require a recognized identity in the target tenant. 

2. A Conditional Access policy is blocking your account

The external company's Entra policies may be evaluating your sign-in and denying it. Microsoft documentation and support discussions show this exact error can occur when the target tenant blocks or disables access for the incoming identity. 

3. Your account is being automatically selected

Sometimes the browser silently sends your signed-in Microsoft 365 work account to the external tenant. The external tenant then evaluates that account and rejects it.

A useful test is:

Open an InPrivate/Incognito window.

Navigate directly to the SharePoint URL.

When asked to sign in, choose"Use another account" if available.

Alternatively test from a browser where you are not already signed into Microsoft 365.