Quartz exception on startup

Question

Quartz.net has been working fine, but suddenly I get the following exception on startup:

Quartz.SchedulerConfigException:
Failure occured during job recovery. ---> 
Quartz.JobPersistenceException: Couldn't store trigger"

This occurs immediately on startup:

Quartz.ISchedulerFactory sf = new StdSchedulerFactory();
schedule = sf.GetScheduler();
schedule.Start();

What do I do now? I can't find any Quartz.net method that 'clears errors', and so I am stuck with the uninitialized class. One option is to just remove all Quartz jobs from the database, but that's ugly and has an impact on existing users.