Hi Matthias thanks for posting in the community! These icons look like they're applied by conditional formatting. If this sounds unintentional I suspect that you may have accidentally created a rule that is now applying to these shapes. To check and view the rule click on the conditional formatting icon in the toolbar at the top of the editor; any created rules will show below.

To remove the rule which will then remove the icon from your shapes simply delete the rule by clicking on the three dots on the right of the rule then choosing the Delete option. 

Please give this a try and refer to this Conditional Formatting and Icon Sets article from our Help Center for more information. Post any other questions below!