I applied the security fixes too, on a standard forum version 3.3 and on 4 beta and I don't get this error when locking a topic.
Did you change anything in your pop_lock.asp? (This is the only file which can give this error.) Maybe the error is caused by an incompatibility with a mod you have installed and which comes with a changed version of this file.
It might also be a cookie problem... Did you try with different cookie settings?
Anyway, to narrow down the problem, you could insert some lines into pop_lock.asp for testing. Find (about line 42)
<%
select case Request.QueryString("mode")
case "CloseTopic"
mLev = cint(chkUser(strDBNTFUserName, Request.Form("Pass")))
and add the following lines after this
'*********test********
response.write "********TEST LOCKING TOPICS********<br>"
response.write "At least one of the following must be ok:<br>"
response.write "mLev (4 = admin, ok): " & mLev & "<br>"
response.write "chkForumModerator with Request.Form(""Name"")(1 = ok): " & _
chkForumModerator(Request.Form("FORUM_ID"), Request.Form("Name")) & "<br>"
response.write "chkForumModerator with Session(strCookieURL & ""userid"")(1 = ok): " & _
chkForumModerator(Request.Form("FORUM_ID"), Session(strCookieURL & "userid")) & "<br>"
response.write "***********************************<br>"
'*********end test********
Hope that helps...
Barbara