Do you have a debug view in your app to trigger an error in sentry? If so, can you trigger the error and see if you are getting any exceptions?
I have faced similar problem while upgrading from 9.0 to 20.10.dev. I have added debug view like this.
def sentry_debug_view(request):
raise this_is_a_test_exception_for_sentry # noqa
When I triggered this view from the browser, there were some exceptions due to the CSRF token which was resolved as mentioned here Issue with CSRF token after sentry upgrade