Reproduction:
<telerik:RadButton ID="RadButton1" runat="server" Text="Submit" EnableEmbeddedSkins="false" EnableEmbeddedBaseStylesheet="false"/>
<telerik:RadCheckBox ID="RadCheckBox1" runat="server" Text="CheckBox" Skin="Default" />
Workaround:
<telerik:RadButton ID="RadButton1" runat="server" Text="Submit" Skin=""/>