Sql – Transaction count after EXECUTE indicates a mismatching number of BEGIN and COMMIT

Question: I have this SQL script:

DECLARE @in_TE_UID varchar(36) 
DECLARE @in_ZO_BETE_Sort int 
DECLARE @in_user int 


SET @in_TE_UID = '9f510440-8828-44ce-bbea-6bc866902262'
SET @in_ZO_BETE_Sort = 0 
SET @in_user = 12435

-- http://stackoverflow.com/questions/884334/tsql-try-catch-transaction-in-trigger
BEGIN TRANSACTION BeforeUpdate;
BEGIN TRY
    IF NOT EXISTS 
    (
        SELECT 1 FROM T_FMS_ZO_Benutzer_TeaserOrder 
        WHERE (T_FMS_ZO_Benutzer_TeaserOrder.ZO_BETE_Status = 1) 
        AND (T_FMS_ZO_Benutzer_TeaserOrder.ZO_BETE_BE_ID = @in_user) 
        AND 
        (
            T_FMS_ZO_Benutzer_TeaserOrder.ZO_BETE_TE_UID = @in_TE_UID 
            OR 
            (
                @in_TE_UID IS NULL 
                AND 
                T_FMS_ZO_Benutzer_TeaserOrder.ZO_BETE_TE_UID IS NULL 
            ) 
        )    
    )
    BEGIN
        INSERT INTO T_FMS_ZO_Benutzer_TeaserOrder 
        (
             ZO_BETE_UID
            ,ZO_BETE_BE_ID
            ,ZO_BETE_TE_UID
            ,ZO_BETE_Sort
            ,ZO_BETE_Status
        )
        VALUES
        (
             NEWID() --<ZO_BETE_UID, uniqueidentifier,>
            ,@in_user --<ZO_BETE_BE_ID, int,>
            ,@in_TE_UID --<ZO_BETE_TE_UID, uniqueidentifier,>
            ,@in_ZO_BETE_Sort --<ZO_BETE_Sort, int,>
            ,1 --<ZO_BETE_Status, int,>
        )
        ;
    END
    ELSE
    BEGIN

        UPDATE T_FMS_ZO_Benutzer_TeaserOrder  
            SET  ZO_BETE_Sort = @in_ZO_BETE_Sort 

        WHERE (T_FMS_ZO_Benutzer_TeaserOrder.ZO_BETE_Status = 1) 
        AND (T_FMS_ZO_Benutzer_TeaserOrder.ZO_BETE_BE_ID = @in_user) 
        AND 
        (
            T_FMS_ZO_Benutzer_TeaserOrder.ZO_BETE_TE_UID = @in_TE_UID 
            OR 
            (
                @in_TE_UID IS NULL 
                AND 
                T_FMS_ZO_Benutzer_TeaserOrder.ZO_BETE_TE_UID IS NULL 
            ) 
        ) 
        ;

    END

END TRY
BEGIN CATCH
    IF @@TRANCOUNT > 0
        ROLLBACK TRANSACTION BeforeUpdate --RollBack in case of Error

    --RETURN


    DECLARE @ErrorMessage NVARCHAR(4000);
    DECLARE @ErrorSeverity INT;
    DECLARE @ErrorState INT;

    SELECT 
        @ErrorMessage = ERROR_MESSAGE(),
        @ErrorSeverity = ERROR_SEVERITY(),
        @ErrorState = ERROR_STATE();

    -- Use RAISERROR inside the CATCH block to return error
    -- information about the original error that caused
    -- execution to jump to the CATCH block.
    RAISERROR (@ErrorMessage, -- Message text.
               @ErrorSeverity, -- Severity.
               @ErrorState -- State.
    );
END CATCH

It runs fine when I run it once.

If afterwards I run a

SELECT * FROM T_FMS_ZO_Benutzer_TeaserOrder

then it timeouts.

If I add with (nolock) then it works.

If I look at the processes in SSMS, I see a locked select with LCK_M_S.

If I execute the same command in code, I always get this error:

Transaction count after EXECUTE indicates a mismatching number of BEGIN and COMMIT statements. Previous count = 0, current count = 1

So really, what's wrong with this code ?