OK Tommy,
I managed to solve the problem.
In your documentation:
" HEADERS: {"Authorization": "Basic {BASE64ENCODED_CLIENT_ID}:{BASE64ENCODED_CLIENT_SECRET}"}
"
You mislead us a bit as the true answer is:
" HEADERS: {"Authorization": "Basic BASE64ENCODED(CLIENT_ID:CLIENT_SECRET)"}
"
Please fix it in - Error in Refreshing with refresh tokens
BR,
Efy