We have a workflow with some external users. The external users are form based authentication users in sharepoint. They can login to sharepoint with a set of user name and password. In the workflow, we assign a flexi task to the external user with his email address. The user received the email for the task, but when he tries to approve the task, an error occurred. In the workflow history detail, the task's owner column was blank. It seems the system couldn't find the user with his email.
Any suggestions are appreciated.