I tried to reproduce the problem you reported, but to no avail. Please compare the attached test sample with your actual project and check if there are differences in the setup as I may have missed something.
You can try if the issue persists after configuring the captcha to use the Session as a storage location for its image, because currently there is a problem with the Cache storage location (set by default) when the page is accessed by multiple users. To do so, please follow the first two steps in this help article
If you are still having difficulties, please try modifying the attached sample in order to recreate your scenario and open a support ticket in order to send it. In case this is not possible, you can also send a simple, fully runnable page that isolates the problem so that I can inspect it locally and provide a more to the point answer.
the Telerik team
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