SQL2012SP1, simple recovery, transaction log full, can't add another, won't grow

SUMMARY:

I have many databases with full transaction logs. They are already in SIMPLE recovery model,
and I cannot add a new log or truncate the existing one, because the transaction log is full due to 'CHECKPOINT'


DETAILS:

(Examining just one of the problematic databases)

I have a SQL Server 2012 SP1 database with a full transaction log.

This database has one data file (750MB) and one log file (850MB)

Compatibility level = SQL Server 2012 (110)

Recovery Model is Simple
Under Recovery Options, Page Verify=CHECKSUM and Target Recovery Time=0

I had 5G of free disk (which should have been enough), and I added more, so now I have 25G free.

My log file growth is set to: "By 10 percent, Limited to 2097152 MB "
(I can't change it - if I try I get an error because the transaction log is full)

I can't do a backup - get an error because the transaction log is full due to 'CHECKPOINT'

I can't add another log file - get an error because the transaction log is full due to 'CHECKPOINT'

Verifying that file#2 is my log file

SELECT file_id, name FROM sys.database_files;

I can't do a truncate only shrink successfully:

DBCC SHRINKFILE (2, TRUNCATEONLY);

- get an error because my log is out of space - with a cascade error that the transaction log is full due to 'CHECKPOINT'

DBCC CHECKDB mostly just gives errors because the transaction log is full due to 'CHECKPOINT'

I can't change the recovery model to full - get an error because the transaction log is full due to 'CHECKPOINT'

I can't change the recovery model to bulk-logged - get an error because the transaction log is full due to 'CHECKPOINT'

From sys.databases,

is_read_only=0, state=0, state_desc=ONLINE, is_in_standy=0
is_cleanly_shutdown=0, is_supplemental_logging_enabled=0
is_read_committed_snapshot_on=0
recovery_model=3, recovery_model_desc=SIMPLE
is_fulltext_enabled=1
is_published=0, is_subscribed=0, is_merge_published=0, is_distributed=0, is_sync_with_backup=0
is_broker_enabled=1, log_reuse_wait=1, log_reuse_wait_desc=CHECKPOINT

I tried doing a manual CHECKPOINT (using the CHECKPOINT cmd in MgmtStudio) and got errors:

SELECTcount(*)FROM fn_dblog (NULL,NULL);

4,521,289

That seems like a lot !?

My ERRORLOGs are full of errors about full transaction logs. I have many databases with this same issue - I've just focused on one, for testing.


DBCC OPENTRAN
gives me no open transactions


However,

select * from sys.dm_tran_database_transactions where database_id = MYDBID

gives me 6 rows, of which 4 have null begin times



I tried using

select request_session_id, * from sys.dm_tran_locks where resource_database_id = MYDBID and resource_subtype ='BULKOP_BACKUP_LOG'

and killing those sessions, but they always come back immediately.

Note: I have seen that SQL2012SP1CU2 has a fix for migrated databases with stuck-open transactions, but this database was created in SQL2012 - it was not upgraded from a prior version.

Thanks for any help or suggestions!