I created custom background job following instruction.
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Created task handler and constructor of task is called every minute. Process method is never called. Also I am getting timeout exception "Orchard.Tasks.BackgroundService - Error while processing background task".
It is interesting that it is possible to add new task, but any query won't work.
I checked table Scheduling_ScheduledTaskRecord and it was locked. I am getting timeout exception in code and SQL Management interface. Lock is released once I turn of site process.