Hopefully to document this for other users:
After a migration of SSRS from OldServer to NewServer, upon attempting to create a new subscription schedule, the error:
"An error occurred within the report server database. This may be due to a connection failure, timeout or low disk condition within the database. (rsReportServerDatabaseError) Get Online Help
Invalid object name 'OldServerTempDB.dbo.ExecutionCache'.
Following the instructions in http://www.sqlservercentral.com/Forums/Topic681805-162-1.aspx#bm1278234
showed that in 2012, like previous versions, the trigger NewReportServerDB.dbo.Schedule.Schedule_UpdateExpiration had a hardcoded reference to OldReportServer which needed manual update, as the automatic migration failed to change it. Selecting the new database in the Reporting Services Configuration Manager also failed to change it.