Some of the other answers did not work for me: It was not possible to create the checkpoint while the db was online, because the transaction log was full (how ironic). However, after setting the database to emergency mode, I was able to shrink the log file:
alter database <database_name> set emergency;use <database_name>;checkpoint;checkpoint;alter database <database_name> set online;dbcc shrinkfile(<database_name>_log, 200);