Thank you for getting back to me and confirming this. I've had this happen a couple of times for me while debugging something and I also tried the same solution, but having a confirmation from a real app helps a lot. I added this info to the documentation: commit 1 and commit 2 and that paragraph will probably be live in the docs by the time you are reading this. Thus, I am closing this one.

Regards,
Marin Bratanov
Progress Telerik

Hi Boštjan,

Have you had a chance to try removing the defer attribute to see if this helps? Do you still experience this issue?

Regards,
Marin Bratanov
Progress Telerik

Hello Boštjan,

The error indicates that the Telerik JS Interop file has not loaded. There is no reason for it to happen only sometimes, apart from some very weird networking issue. At this point I can only suggest you try removing the defer attribute of our script tag to see if this helps - the attribute lowers the execution priority of the script and perhaps some other scripts that require it now sometimes manage to run before our file is parsed. Do let me know if this helps.

Thanks,
Marin Bratanov
Progress Telerik