Campaign creation using optimizer: Lock error while changing campaigns

What causes this issue?

While data was being transferred after an optimization run, the
following happened:
For access to a campaign that was changed during optimization, no lock
could be set. The campaign was obviously 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?

Restart the transaction. Make sure that no campaigns are locked by
other users.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.