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The Isabel Card reader can be blocked by another process or it may be a technical malfunction.
To solve this problem follow these steps:
  1. Remove the Isabel SmartCard from the reader.
  2. Disconnect the card reader and plug it into another USB port. 
  3. Unplug the USB cable from the card reader itself, then reconnect it.
  4. Now, insert your Isabel SmartCard and try to logon to Isabel 6.
 

If the issue cannot be resolved using the aforementioned guidelines, then a resolution may require additional, more complex steps.