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OK, even now I realized the solution by myself.
First, I found the use of an editor for the description field at line 84 in the file \administrator\components\com_community\views\groups\tmpl\edit.php
With this info I had the idea to choose the editor plugin again.
My choice was plg_editors_jce and at least the style sheets for the JCE editor was extended by me to reach the full set of styles for UIKit.
This action could be an explaination but I'm not sure.
Nevertheless to switch to another editor and back to the JCE in the combo selection box of your configuration page helped and solved my problem with the missing description field.