I would check the internal memory for errors.
If you got a windows machine, connect the reader. The right click on the device and choose "Properties" (? My windows is German. The option might be called something else). In the property window go to the "Extras" tab and use the error check.
The memory can get corrupted when disconnecting the device without using "safely disconnect". Some reader react to corrupted memory parts with write protecting the memory.
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