I researched the past Community conversations and internal resources, however, I didn't find a similar report which is adding the HubSpot tracking code into a WordPress page causes 503 error.
If you have a paid subscription (I see you put "Professional" label), I'd highly recommend reaching out to the Technical Support team via phone call and share your page URL and show what is the exact behaviour you're seeing.
If you're using Free HubSpot account, could you please confirm the following?
1. Could you please check your browser and OS are on the latest version?
2. Do other people in your team also observe 503 error when accessing this WordPress page?