UPDATE:
I found that if I remove the [SSI0700\SQLEXPRESS] from the INSERT statement that I now get this error:


Invalid object name 'WebApp.dbo.UserInformation'.
Transaction count after EXECUTE indicates that a COMMIT or ROLLBACK TRANSACTION statement is missing. Previous count = 0, current count = 1.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.



This is my edit of the stored procedure command as it currently is (my changes start at --Xadeu edit--):

set ANSI_NULLS ON
set QUOTED_IDENTIFIER ON
go

ALTER PROCEDURE [dbo].[aspnet_Membership_CreateUser]
@ApplicationName nvarchar(256),
@UserName nvarchar(256),
@Password nvarchar(128),
@PasswordSalt nvarchar(128),
@Email nvarchar(256),
@PasswordQuestion nvarchar(256),
@PasswordAnswer nvarchar(128),
@IsApproved bit,
@CurrentTimeUtc datetime,
@CreateDate datetime = NULL,
@UniqueEmail int = 0,
@PasswordFormat int = 0,
@UserId uniqueidentifier OUTPUT
AS
BEGIN
DECLARE @ApplicationId uniqueidentifier
SELECT @ApplicationId = NULL

DECLARE @NewUserId uniqueidentifier
SELECT @NewUserId = NULL

DECLARE @IsLockedOut bit
SET @IsLockedOut = 0

DECLARE @LastLockoutDate datetime
SET @LastLockoutDate = CONVERT( datetime, '17540101', 112 )

DECLARE @FailedPasswordAttemptCount int
SET @FailedPasswordAttemptCount = 0

DECLARE @FailedPasswordAttemptWindowStart datetime
SET @FailedPasswordAttemptWindowStart = CONVERT( datetime, '17540101', 112 )

DECLARE @FailedPasswordAnswerAttemptCount int
SET @FailedPasswordAnswerAttemptCount = 0

DECLARE @FailedPasswordAnswerAttemptWindowStart datetime
SET @FailedPasswordAnswerAttemptWindowStart = CONVERT( datetime, '17540101', 112 )

DECLARE @NewUserCreated bit
DECLARE @ReturnValue int
SET @ReturnValue = 0

DECLARE @ErrorCode int
SET @ErrorCode = 0

DECLARE @TranStarted bit
SET @TranStarted = 0

IF( @@TRANCOUNT = 0 )
BEGIN
BEGIN TRANSACTION
SET @TranStarted = 1
END
ELSE
SET @TranStarted = 0

EXEC dbo.aspnet_Applications_CreateApplication @ApplicationName, @ApplicationId OUTPUT

IF( @@ERROR <> 0 )
BEGIN
SET @ErrorCode = -1
GOTO Cleanup
END

SET @CreateDate = @CurrentTimeUtc

SELECT @NewUserId = UserId FROM dbo.aspnet_Users WHERE LOWER(@UserName) = LoweredUserName AND @ApplicationId = ApplicationId
IF ( @NewUserId IS NULL )
BEGIN
SET @NewUserId = @UserId
EXEC @ReturnValue = dbo.aspnet_Users_CreateUser @ApplicationId, @UserName, 0, @CreateDate, @NewUserId OUTPUT
SET @NewUserCreated = 1
END
ELSE
BEGIN
SET @NewUserCreated = 0
IF( @NewUserId <> @UserId AND @UserId IS NOT NULL )
BEGIN
SET @ErrorCode = 6
GOTO Cleanup
END
END

IF( @@ERROR <> 0 )
BEGIN
SET @ErrorCode = -1
GOTO Cleanup
END

IF( @ReturnValue = -1 )
BEGIN
SET @ErrorCode = 10
GOTO Cleanup
END

IF ( EXISTS ( SELECT UserId
FROM dbo.aspnet_Membership
WHERE @NewUserId = UserId ) )
BEGIN
SET @ErrorCode = 6
GOTO Cleanup
END

SET @UserId = @NewUserId

IF (@UniqueEmail = 1)
BEGIN
IF (EXISTS (SELECT *
FROM dbo.aspnet_Membership m WITH ( UPDLOCK, HOLDLOCK )
WHERE ApplicationId = @ApplicationId AND LoweredEmail = LOWER(@Email)))
BEGIN
SET @ErrorCode = 7
GOTO Cleanup
END
END

IF (@NewUserCreated = 0)
BEGIN
UPDATE dbo.aspnet_Users
SET LastActivityDate = @CreateDate
WHERE @UserId = UserId
IF( @@ERROR <> 0 )
BEGIN
SET @ErrorCode = -1
GOTO Cleanup
END
END

INSERT INTO dbo.aspnet_Membership
( ApplicationId,
UserId,
Password,
PasswordSalt,
Email,
LoweredEmail,
PasswordQuestion,
PasswordAnswer,
PasswordFormat,
IsApproved,
IsLockedOut,
CreateDate,
LastLoginDate,
LastPasswordChangedDate,
LastLockoutDate,
FailedPasswordAttemptCount,
FailedPasswordAttemptWindowStart,
FailedPasswordAnswerAttemptCount,
FailedPasswordAnswerAttemptWindowStart )
VALUES ( @ApplicationId,
@UserId,
@Password,
@PasswordSalt,
@Email,
LOWER(@Email),
@PasswordQuestion,
@PasswordAnswer,
@PasswordFormat,
@IsApproved,
@IsLockedOut,
@CreateDate,
@CreateDate,
@CreateDate,
@LastLockoutDate,
@FailedPasswordAttemptCount,
@FailedPasswordAttemptWindowStart,
@FailedPasswordAnswerAttemptCount,
@FailedPasswordAnswerAttemptWindowStart )

IF( @@ERROR <> 0 )
BEGIN
SET @ErrorCode = -1
GOTO Cleanup
END

--Xadeu Edit: Adding the new user's username into the UserInformation table here
--so that if anything in the User Creation Process fails we can rollback everything
--at once.

INSERT INTO WebApp.dbo.UserInformation(UserName) VALUES (@UserName)

--INSERT INTO [SSI0700\SQLEXPRESS].WebApp.dbo.UserInformation(UserName) VALUES (@UserName)
--select * from WebApp..UserInformation

IF( @@ERROR <> 0 )
BEGIN
SET @ErrorCode = -1
GOTO Cleanup
END

--DONE WITH Xadeu EDIT--

IF( @TranStarted = 1 )
BEGIN
SET @TranStarted = 0
COMMIT TRANSACTION
END


RETURN 0

Cleanup:

IF( @TranStarted = 1 )
BEGIN
SET @TranStarted = 0
ROLLBACK TRANSACTION
END

RETURN @ErrorCode

END