Marin, thanks for your reply. Please see my answers below:
Can you confirm that the repeater renders as expected outside of a RadWindow?
>>>Yes, the repeater renders normally outside of a RadWindow.
make sure the page has a valid doctype. Xhtml 1.0 Transitional is one of the most forgiving and easiest to satisfy. make sure the page HTML is valid.
>>>I tried <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> and <!DOCTYPE html>, but got the same results
make sure that no CSS is interfering with the repeater
>>>no CSS interference
try with different data, if the original data contains some (invalid) HTML, it could break the repeater. Judging from the screenshot, the second record's Code field may be causing the problemtry
>>>I tried just put simple characters such ABC on each column, but still got the same results. The rows which have shifted columns are random. Sometimes it's row 3, sometiems it's row 9. There are not just one row with the issue.
upgrading to the latest Telerik controls version to see if this helps. The version you have is quite old and does not support modern browsers
>>>I installed the latest version 2015.1.401.35 but still got the same issue