Will,

I had the same problem a while ago, and I don't remember exactly what it was (and unfortunately I did not document it anywhere) - but I think that YAF modified one of the DNN Stored procedures (RemoveUser?), and deleting this procedure and re-creating it from the original DNN script did the trick.

Hope that helps...

Happy DNNing!
Michael

Aha!  I should've thought to search through the sprocs.  You were correct about this.  The RemoveUsers procedure was modified.  This is a core code change.  Had I known this was happening, I wouldn't have ever even tried using this module.  :(  

It was the only sproc that had the term yaf in it.  I used these queries to find it: 

SELECT OBJECT_NAME(object_id)
    FROM sys.sql_modules
    WHERE OBJECTPROPERTY(object_id, 'IsProcedure') = 1
    AND definition LIKE '%yaf%'

SELECT OBJECT_NAME(id) 
    FROM SYSCOMMENTS 
    WHERE [text] LIKE '%yaf%' 
    AND OBJECTPROPERTY(id, 'IsProcedure') = 1 
    GROUP BY OBJECT_NAME(id)

SELECT ROUTINE_NAME, ROUTINE_DEFINITION
    FROM INFORMATION_SCHEMA.ROUTINES 
    WHERE ROUTINE_DEFINITION LIKE '%yaf%' 
    AND ROUTINE_TYPE='PROCEDURE'

I used a clean instance of DNN 9.12 (same version) to drop and recreate the sproc: 

/****** Object:  StoredProcedure [dbo].[RemoveUser]    Script Date: 8/31/2023 9:07:40 AM ******/
DROP PROCEDURE [dbo].[RemoveUser]
GO

/****** Object:  StoredProcedure [dbo].[RemoveUser]    Script Date: 8/31/2023 9:07:40 AM ******/
SET ANSI_NULLS ON
GO

SET QUOTED_IDENTIFIER ON
GO

CREATE PROCEDURE [dbo].[RemoveUser]
    @UserID        int,
    @PortalID   int
AS
    IF @PortalID IS NULL
        BEGIN
            -- Delete SuperUser
            DELETE FROM dbo.Users
                WHERE  UserId = @UserID
        END
    ELSE
        BEGIN
            -- Remove User from Portal
            DELETE FROM dbo.UserPortals
                WHERE  UserId = @UserID
                 AND PortalId = @PortalID
            IF NOT EXISTS (SELECT 1 FROM dbo.UserPortals WHERE  UserId = @UserID)
                -- Delete User (but not if SuperUser)
                BEGIN
                    DELETE FROM dbo.Users
                        WHERE  UserId = @UserID
                            AND IsSuperUser = 0
                    DELETE FROM dbo.UserRoles
                        WHERE  UserID = @UserID
                END
            ELSE
                BEGIN
                    DELETE ur FROM dbo.UserRoles ur
                        INNER JOIN dbo.Roles r ON r.RoleID = ur.RoleID
                        WHERE  UserID = @UserID AND r.PortalID = @PortalID
                END
        END
GO

These are superusers I'm attempting to permanently remove, so far.  They're already marked as deleted, but I still can't use the option in the Users view to "Permanently Delete."  I get a new error, listed below.  However, an interesting thing to note is that running the purge- user prompt command doesn't display any error at all.  If I took that for it's word, the same user accounts I'm trying to delete are deleted, but they're still there.  

ERROR MESSAGE

Message:Internal Query Processor Error: The query processor encountered an unexpected error during execution (HRESULT = 0x80040e19).

StackTrace:

   at System.Data.SqlClient.SqlConnection.OnError(SqlException exception, Boolean breakConnection, Action`1 wrapCloseInAction)
   at System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj, Boolean callerHasConnectionLock, Boolean asyncClose)
   at System.Data.SqlClient.TdsParser.TryRun(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj, Boolean& dataReady)
   at System.Data.SqlClient.SqlCommand.FinishExecuteReader(SqlDataReader ds, RunBehavior runBehavior, String resetOptionsString, Boolean isInternal, Boolean forDescribeParameterEncryption, Boolean shouldCacheForAlwaysEncrypted)
   at System.Data.SqlClient.SqlCommand.RunExecuteReaderTds(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, Boolean async, Int32 timeout, Task& task, Boolean asyncWrite, Boolean inRetry, SqlDataReader ds, Boolean describeParameterEncryptionRequest)
   at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method, TaskCompletionSource`1 completion, Int32 timeout, Task& task, Boolean& usedCache, Boolean asyncWrite, Boolean inRetry)
   at System.Data.SqlClient.SqlCommand.InternalExecuteNonQuery(TaskCompletionSource`1 completion, String methodName, Boolean sendToPipe, Int32 timeout, Boolean& usedCache, Boolean asyncWrite, Boolean inRetry)
   at System.Data.SqlClient.SqlCommand.ExecuteNonQuery()
   at PetaPoco.Database.<>c.<ExecuteNonQueryHelper>b__353_0(IDbCommand c)
   at PetaPoco.Database.CommandHelper(IDbCommand cmd, Func`2 cmdFunc)
   at PetaPoco.Database.ExecuteNonQueryHelper(IDbCommand cmd)
   at PetaPoco.Database.ExecuteInternal(CommandType commandType, String sql, Object[] args)
   at DotNetNuke.Data.PetaPoco.PetaPocoHelper.ExecuteNonQuery(String connectionString, CommandType type, Int32 timeoutSec, String sql, Object[] args)
   at DotNetNuke.Data.SqlDataProvider.ExecuteNonQuery(String procedureName, Object[] commandParameters)
   at DotNetNuke.Security.Membership.AspNetMembershipProvider.RemoveUser(UserInfo user)

This means that there are likely MORE core code changes that were made.  Ugh...