When a user sends a task request, the recipient clicks "accept" and a popup entitled "Accepting Task" should appear giving them the option to either "edit the response before sending" or "send the response now".
One of our users (User1) is getting this prompt for all users but one (User2). When User1 receives a task request from User2 and clicks accept, the task is accepted immediately and appears in User1's task list as unread. The same thing happens when User1 declines tasks sent by User2; no prompt, it just declines so User1 cannot give a reason as to why it's been declined.
As the SysAdmin, I cannot work out why this is happening and why it's only happening between these users. If User2 sends me a task request and I click accept/decline, I get the prompt, as does anyone else in the company.
What would cause something like this to happen?
Could it be a corrupt installation of Outlook or the mailbox itself?
Who's Outlook/mailbox could be the cause of the problem?