Problem with combination of isr_mbx_send and os_mbx_wait

Good evening,

i'm having problems with sending a mail from an interrupt to a task. I'm using the LPC1769 (Cortex-M3). The os_mbx_wait-function just waits forever, although the isr_mbx_send() is multiply called (checked with debugger)! If I send a mail from another task it works well - seems as if the isr_mbx_send() is not working correctly.

My Code is looking like this:

..
os_mbx_declare(pwm_in_mail, 20);

void pwm_in_init(void)
{
    os_mbx_init(pwm_in_mail, sizeof(pwm_in_mail));
    os_tsk_create (pwm_in_task, 2);
    ..
}

void EINT3_IRQHandler()
{
    U32 mail;
    ..
    mail = (channel->id << 16) | channel->pulse_width;

    if(isr_mbx_check(pwm_in_mail) != 0)
        isr_mbx_send(pwm_in_mail, (void*)mail);
}

__task void pwm_in_task(void)
{
    U32 mail_data, channel, time;

    for(;;)
    {
        os_mbx_wait(pwm_in_mail, (void*)&mail_data, 0xFFFF);
        channel = mail_data >> 16;
        time = mail_data & 0xFFFF;

        if(channel == PWM_IN_CHANNEL_1)
            pwm_set(PWM_CHANNEL_1, time);
    }
}

thx for your help!
regards, Sascha Albrecht