Hi Alexander,
The issue of
'Security alert occurs while using RichTextBox control in HTTPS environment' has been fixed. The fix for this defect is available in our Development build v6.2.0.13.
Please refer to the below link for the weekly development build updates.
http://www.syncfusion.com/downloads/product/build.aspxPlease let me know if you have any questions.
Regards,
Manopriya