Cannot share issue with JIRA user; JIRA server returns an error

[Sebastien]

We have about 30 users in JIRA and only one has this issue. His permissions are an exact copy of other users and still, trying to share an issue to this user results in the following :

1- Keying the first letter of the username returns a pop-up error message stating : The JIRA server was contacted but has returned an error response. We are unsure of the result of this operation.

2- Pasting full email adress returns : The value XXX.XXX.XXX@XXX.XXX.com is invalid. Any other emails I paste in here do work. If I truncate the email adress to XXX.XXX@XXX.XXX.com, the same happens. Any part of his name return an error. No special character; all letters.

The same behavior happens if I try to assign an issue to this user.

What troubles me is that he is assigned to 4 issues and has commented 7 times on issues.

Your help is much appreciated 

Many thanks

Seba

[Sebastien]

Update : using username works, but full name does not. Works for all other users.