I met the error: "The transaction log for database '...' is full due to 'ACTIVE_TRANSACTION' while deleting old rows from tables of my database for freeing disk space. I realized that this error would occur if the number of rows to be deleted was bigger than 1000000 in my case. So instead of using 1 DELETE statement, i divided the delete task by using DELETE TOP (1000000).... statement.
For example:
instead of using this statement:
DELETE FROM Vt30 WHERE Rt < DATEADD(YEAR, -1, GETDATE())
using following statement repeatedly:
DELETE TOP(1000000) FROM Vt30 WHERE Rt < DATEADD(YEAR, -1, GETDATE())