Running Tasks Gives "The thread is no longer running"

We are using Kentico 9.0 and are trying to rebuild some indexes. I check the tasks tab on the admin area under the Smart Search app but found the tasks have not executed. When I click the "Process Tasks" button and click the "Task processor is running. Watch progress here" link I get this message:

The thread is no longer running

We are using virtual storage on Amazon S3. I checked and deleted any lock files there. There are subdirectories for each smart search index.

I appreciate any ideas.