investigate halt of scheduling for one of many tasks in collector's scheduler

Affected is the current HEAD of the master branch.

The scheduling of updateindex was stopped w/o any exception/warning whatsoever.

Positive about this is that the other scheduled tasks were not affected.

First steps:

1. add some logging to threads of the thread-pool
2. maybe, use other ways to study this behavior
3. implement a solution based on the facts found in the steps above.