When the transaction log becomes nearly full, it may be impossible to use standard procedures to dump transactions and reclaim space.
The system administrator should monitor the log size and perform regular transaction log dumps (in addition to regular database dumps) to avoid this situation. Set up a threshold stored procedure that notifies you (or dumps the log) when the log reaches a certain capacity. See System Administration Guide: Volume 2 > Managing Free Space with Thresholds. SAP also suggests that, to shorten the time required to dump and load the database, dump the transaction log immediately prior to performing a full database dump.
You can also monitor the space used in the log segment manually using sp_helpsegment, as described in System Administration Guide: Volume 2 > Creating and Using Segments.