The bug has been reported in bugtracker, but is still unsolved. Here's the error message when trying to add an advertisment:
Server Error
There was a server error processing your request. We apologize.
Take me back where I was (before the error)
can't read "ad_id": no such variable
    while executing
"classified-ads::get_attribute_values -item_id $ad_id -item_id_element ad_id"
    invoked from within
"set form_values [classified-ads::get_attribute_values -item_id $ad_id -item_id_element ad_id]"
    ("uplevel" body line 11)
    invoked from within
"uplevel #5 {
	# Get extra custom widgets, if any
	classified-ads::widgets::get_widget_params  -content_type "ca_ad"  -form_id ad  -keyword_id $keywor..."
    ("uplevel" body line 1)
    invoked from within
"uplevel 1 $args"
    (procedure "ad_page_contract_eval" line 2)
    invoked from within
"ad_page_contract_eval uplevel #$level $on_request"
    (procedure "ad_form" line 387)
    invoked from within
"ad_form -name ad -form {
	ad_id:key
	{return_url:text(hidden)	{label "return url"}
					{value $return_url} }
	{keyword_id:integer(hidden)	{label "Key..."
    ("uplevel" body line 39)
    invoked from within
"uplevel {
    	  ad_page_contract {
    Create a Classified Ads Ad
    @author Deds Castillo (mailto:deds@infiniteinfo.com)
    @modified-by Roberto Me..."
    (procedure "code::tcl::/usr/local/www/timedeskblog.com/openacs-5-2-2/pac..." line 2)
    invoked from within
"code::tcl::$__adp_stub"
    invoked from within
"if { [file exists $__adp_stub.tcl] } {
      # ensure that data source preparation procedure exists and is up-to-date
      adp_init tcl $__adp_stub
..."
    ("uplevel" body line 3)
    invoked from within
"uplevel {
    if { [file exists $__adp_stub.tcl] } {
      # ensure that data source preparation procedure exists and is up-to-date
      adp_init t..."
    (procedure "adp_prepare" line 2)
    invoked from within
"adp_prepare "
    (procedure "template::adp_parse" line 30)
    invoked from within
"template::adp_parse [file root [ad_conn file]] {}"
    (procedure "adp_parse_ad_conn_file" line 5)
    invoked from within
"$handler"
    ("uplevel" body line 2)
    invoked from within
"uplevel $code"
    invoked from within
"ad_try {
                $handler
            } ad_script_abort val {
                # do nothing
            }"
    invoked from within
"rp_serve_concrete_file [ad_conn file]"
    (procedure "rp_serve_abstract_file" line 60)
    invoked from within
"rp_serve_abstract_file "$root/$path""
    ("uplevel" body line 2)
    invoked from within
"uplevel $code"
    invoked from within
"ad_try {
            rp_serve_abstract_file "$root/$path"
            set tcl_url2file([ad_conn url]) [ad_conn file]
            set tcl_url2path_info..."