I had this problem with a certain theme, to fix the error:
In your Admin CP under Styles & Template select Search In Templates...
Search for: value="$session[sessionhash]"
In every template this occurs in add this line directly after the line containing the above, if it doesn't exist already:
<input type="hidden" name="securitytoken" value="$bbuserinfo[securitytoken]" />
Save the template.
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I did this with the header template and it solved my searching problem
it only affects certain skins like webchat.
In your Admin CP under Styles & Template select Search In Templates...
Search for: value="$session[sessionhash]"
In every template this occurs in add this line directly after the line containing the above, if it doesn't exist already:
<input type="hidden" name="securitytoken" value="$bbuserinfo[securitytoken]" />
Save the template.
---------------------
I did this with the header template and it solved my searching problem
it only affects certain skins like webchat.