Not sure about the 404 error, maybe just an authentication issue. Or permissions in your Azure directory.
To monitor a pipeline or ADF in general I would suggest using PowerShell. There are loads of cmdlets to do things beyond the Azure Portal UI, including set a time slice status.
For example, to check what's currently 'In Progress' in your factory do something like this...
Import-Module Azure
#Params...
$AzureUser = "" # <<< enter username
$AzurePass = "" #<<< enter password
$AzureSubscription = "" # <<< enter subscription name
$ResourceGroup = "" # <<< enter resource group name
#Create credential
$SecurePassword = ConvertTo-SecureString $AzurePass -AsPlainText -Force
$PSCredential = New-Object System.Management.Automation.PSCredential ($AzureUser, $SecurePassword)
#Create Azure Connection
Login-AzureRmAccount -Credential $PSCredential | Out-Null
#Set context for subscription
$SubId = Get-AzureSubscription `
-SubscriptionName $AzureSubscription | SELECT SubscriptionId
Set-AzureRmContext -SubscriptionId $SubId.SubscriptionId | Out-Null
#Get ADF details
$ADFName = Get-AzureRmDataFactory `
-ResourceGroupName $ResourceGroup | SELECT DataFactoryName
Get-AzureRmDataFactoryActivityWindow `
-DataFactoryName $ADFName.DataFactoryName `
-ResourceGroupName $ResourceGroup `
| ? {$_.WindowState -eq "InProgress"}
Here's a link to the other cmdlets.
https://docs.microsoft.com/en-us/powershell/resourcemanager/azurerm.datafactories/v2.5.0/azurerm.datafactories
Hope this helps.