It is not possible to read the open items for contract &1

What causes this issue?

This can be due to a number of reasons, for example:
The account for this contract is being processed by another user

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Tray again at a later date.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.