To get rid of the error, logout and login again, so the user selection setting in bulk user actions is lost.
In Bulk User action, when selecting more than 1000 users the ORA-01795 errors appears.
Thi issue as a direct relation with MDL-27071
Oracle cannot cope withan IN operator with more than 1000 conditions in it
User bulk selection results in Orable database errors when more than 1000 users are selected