when exporting as animated gifs unwanted circles form Symbols appears

Hello,

When I export my simple animation based on Symbols as animated Gif, unwanted circles appears (black outlines). I assume those circles indicate where the Symbols (grafics) are, but should not be visible in the preview and  the final outcome.

Please, anybody knows how I can get rid off the circles? I assume it is some configuration issue I am not aware of.

Many thanks!

Katerina