Hi Marja and Peter,
According to the information provided so far, here you are some more details on the matter.
About jQuery, to use jQuery library you should always consider using the same jQuery version fetched by the Telerik.Web.UI assembly. Programmatic interference with the controls using a different version of the JS library may lead to such unexpected behavior. You can find more troubleshooting information and details related to jQuery in this documentation section - Using jQuery
As of the issue regarding the validation controls, it is a well known matter, related to ASP.NET Unobtrusive validation, you can find more details about it in this help article - jQuery Troubleshooting
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