A Subset/Privacy Run has been started, but transactions on the database need to be stopped to perform maintenance.
Subset/Privacy runs can be paused (Both when executed from the interface, and when executed through Runtime. After this, currently running processes will finish, then the run will stop making transactions, and new processes will not be started. You can now perform maintenance on your database.
During Subset/Privacy runs temporary tables are generated and used by the database. If you alter or delete these while the run is paused and then continue the run, the run will crash.