There are a lot of possibilities, especially when something as big as an upgrade occurs it could mean a move to another instance, and then previous rules applied to say email or firewalls or spam filters may not apply from a Recipient point of view. Have you tried sending something to yourself to reproduce the issue?

I typically start with sending to myself to verify the email notifications are coming through, checking Junk and Spam if they do not, then checking the DocuSign Trust website to verify there are not any email outages. I then proceed to open a case while in parallel checking with my internal IT to verify something is happening inside the network. Many times this will lead to information about emails being blocked or filtered and rules that need to be applied so these emails are not rejected.

Here is a list of items to check via a DocuSign article that might help...

https://support.docusign.com/en/articles/Why-aren-t-my-signers-receiving-DocuSign-Notification-emails