Outlook 2007 Not Playing Nicely With 2003

[bobber205]

A user of 2007 Outlook is unable to look at the calendars of a 2003 user.
The 2003 user has sent a "allow this user" email and everything to the 2007 user.

Unfortunately the 2007 user has trouble when she tries to open the 2003 user's calendar. Outlook keeps saying "This user does not have a (mail server something like that can't remember exactly) on the server".

I keep trying over and over to open the 2003 user's calendar. After about 20 times, it said "Do you want to request permission?". Ugh. I said yes and keep trying. The same server related error keeping appearing.

What could this be caused by?

 

[BlueWeasel]

Is it asking about an Exchange server perhaps?

 

[bobber205]

It happened again this morning after a fresh boot up. I tried moving it (outlook.ost) to the desktop b/c I wanted to make a fresh copy but it said part of the was locked by another application.

I was like WTF. No apps are open fool!
So I rebooted the machine myself. Outlook decided to start up. Also I was able to make a copy of the file so if it happens again I can force delete it and replace it with the copy.

Any ideas on what's going on? Nobody else in the office is having this problem.