Booking could not be added

Hi!

My company use time tracker plugin, and we really like it.
However we get an error random when want to stop timer: Booking could not be added.
We use bitnami redmine with sql server, if I delete the time tracker data from db, after everything is ok, the start/stop works fine, same issue, same assignee. Could you help me what could cause this? Where can I find the time tracker logs?

Thanks
