IBM Known limitation: You cannot add people to a matter request if there are more than 1000 people selected in the clipboard. - United States  PRODUCT DOCUMENTATION

ABSTRACT
 If you try to add people to a matter request when there are more than 1000 people in the clipboard, the following error will occur:
An unexpected error occurred.
Contact your system administrator.
ICWPA1001E 

CONTENT
To work around the limitation, when you add people from the clipboard, add fewer than 1000 people at one time.
How to resolve "An unexpected error occurred. Contact your system administrator. ICWPA1001E" 

When trying to add people to a matter request when there are more than 1000 people in the clipboard, the following error is reported:

An unexpected error occurred. Contact your system administrator. ICWPA1001E"
To work around the limitation, when you add people from the clipboard, add fewer than 1000 people at one time.