Error in task. Object reference not set to an instance of an object.

Good morning,

I have an issue! I did an import/export of a Nintex form from SharePoint 2013 to SharePoint 2019. In my workflow, in a Flexi task, where some people are supposed to receive an email, I have this error: Error in task. Object reference not set to an instance of an object.

For your information, the people assigned to receive the email are put through variables.

What I have already done:

1. Put my name hardcoded, so see if it was a problem of domain/username. → did not receive any email
2. Update the variable, by re-saving it, to update the form → did not work
3. Re-create a flexi-task to see, if it was an error linked to it → did not work

If anybody, have encountered the same issue in the past, and found a solution, please let me know. It will be really appreciated.

Thank you.