Hi Jag,
We are able to reproduce the issue as you mentioned. We noticed that whenever more than one RichTextBox control placed inside a single page throws error. To workaround this issue you have to use the Html Frameset. Attached is our sample for your references.
http://websamples.syncfusion.com/samples/Tools.Web/6.1.0.34/F70704/main.htmIn our sample, we placed two RichtextEditor on different page and it was invoked by common page using HTML Frameset.We couldn''t found any other workaround other than HTML Frameset.
Let us know if you cannot use our workaround in your scenario for any reason.
Thanks for using Syncfusion products.
Regards,
Vinoth