User Control raise error unknown server tag uc1:

Hi
I faced with a strange behaviour in asp.net, visual studio, a help i s lot appreciated.
I have a control, i add inside an insert template of a form view, the form view is inside an update panel.
when i change a litle bit of form view it hide all the form and display error (unknown server tag uc1:DayHours30mnt)
i have added the control above page 
<%@ Register src="../Controls/DayHours30mnt.ascx" tagname="DayHours30mnt" tagprefix="uc1" %> and the form is added
<uc1:DayHours30mnt ID="DayHours30mnt1" runat="server" /> it happen in all pages of website, but it compile and work corectly, and when i close and open it shows corectly, but by typing only one character it hides everything and show this error.
I do my work, but bilions of  times i have to close and open it.
should i defin ...