Same problem here, getting bad requests error messages all over my integrations.
Even a simple create deal using Zapier official Hubspot CRM integration stopped working:
Looks likes some issue related with array fields (associated contacts, associated deals, etc...)
To simulate the problem on Zapier:
1 - Create a Zapier with some trigger (in my case I used Zapier Push)
2 - Create a second step Hubspot CRM > Create Deal
3 - On edit template, specify fields like deal name, deal stage, etc. Important is to include an associated contact to simulate the error, like the screenshot above.
4 - Try to test this step, it will genarete an error:
"We had trouble sending your test through. Please try again. Error: api.hubapi.com returned (400) Bad Request and said "Invalid input JSON on line 1, column 36: Invalid JSON input: Can not deserialize instance of LONG out of START_ARRAY token at line 1, column 36""
This is just an example, it is generating different error messages all over my integration that has any associated field (array).
I kindly request Hubspot API dev guys to take a look on this.