This specific issue happens with activity type code 2 which is form fill out activity especially dealing with downloading attributes. Few other activity types seems to be working fine. We use Snaplogic as a middleware.. not python..Any pointers plz?
Likely you're misinterpreting the inner form payload as JSON, when it's actually PHP object serialization format.
Also, this is in the wrong space. You need to move the thread to https://nation.marketo.com/t5/Products/ct-p/products which is the correct support area.