AWS authentication access is currently not implemented for thumb_destination.
Alternatively you can create a separate task with output=thumbnail to use this
feature:
To quote our API documentation:
Instead of using thumb_ parameters within <format>, you can specify
separate encoding task with output = thumbnail:
<format>
<!– Format fields –>
<output>thumbnail</output>
<time>[Time]</time>
<width>[Width]</width>
<height>[Size]</height>
<!– Destination fields –>
<destination>[DestFile]</destination>
</format>