This problem is solved.
The problem was in a JSTL custom tag library's for-loop !!
How I solved(debugging JSP): I made different copies of problematic file and included each of them in definition JSP. Each of those problematic JSP files are edited in different areas as follows. If we do so, the error will be in only one/some JSP and so it is easy to find out the exception.
Say:
problematic-ORIGINAL.jsp
<jsp:for...></jsp>
<c:...>..</c>
<a href="<c:..../>"> ... </a>
problematic-1.jsp
<c:...>..</c>
problematic-2.jsp
<jsp:for...></jsp>
problematic-3.jsp
<a href="<c:..../>"> ... </a>
And then include 1,2 and 3 in definition JSP. Thus I could find the prob was in 2nd one. Enjoy coding ..