The specified string is not in the form required for an e-mail address


We have created a workflow, that would trigger an approval when a user is added to a specified group.

But when the approver is clicking the approve button, he/she gets this error:


We have several other workflows doing the exact same thing but for other groups/other approvers, that doesn't give us this error.

Can anyone give me a hint to where to look to solve this issue?