Have you deleted users from the UME datasource or change the LDAP directory structure?
No, no user has been deleted or yes, but it cannot be recreated.
If you know that one of the substitutes of the affected user has been deleted, try to recreate it. It is not recommended to delete users, as the UME functionality may break. Instead, it is a better option to disable the users.
All users on the system involved in BPM scenarios, like potential owners, business process administrators and so on are internally stored in BC_BPEM_UM_MAP DB table. When a user is deleted in UME, there is no notification mechanism so that the corresponding entry in BC_BPEM_UM_MAP table is removed, so a record for the deleted user is still present in that table. If you don't know if some BPM related user has been deleted, you can try the following:
This SQL will provide you with a list of all users the BPM is aware of. Then, you should go to the UME datasource to check if that users are present or no.