Hi Tomasz,
Thanks for contacting Syncfusion support.
We were able to reproduce the reported issue on our end.
We suspect that the reported issue might have occurred because the script might not be referred properly in your master page.
In your Master page layout, please check if you have referred any of the script files more than once (inside ScripManager or Head section). In that case, there might be a conflict in script reference.
If you still face any such difficulties, then please let us know. We’ll be happy to assist you.
Regards.
Deepa L.