Getting error code 401013 while trying to get an upload pre-signed URI using REST API Call

Hi,

We use Adobe Pdf services and recently switched to the REST API version. We used Adobe Pdf Services API usage and followed 5 steps. Everything was working fine until today when we started to receive error code 401013 with the error message "OAuth token is not valid". The error message is returned when we make the call from step 1 API. Below is the code that does it - 

step1_headers = {
'x-api-Key': '<XXXXX>',
'Authorization': 'Bearer <XXXX>',
# Already added when you pass json= but not when you pass data=
# 'Content-Type': 'application/json',
}

step1_data = {
'mediaType': 'application/pdf',
}

step1_response = requests.post('https://pdf-services.adobe.io/assets', headers=step1_headers, json=step1_data)

Does anyone experience this? How to resolve this?