Ad view counting broken

Since an upgrade of D5->D6, ad no longer maintains view statistics correctly. The invisible view counter image does not load, and the alt text is displayed instead. The ads display correctly and click throughs are also counted correctly.

Curiously the charting feature of reports does not work either- this may or may not be relayed. I have disabled ad file caching- this had no effect. The server also has memcached installed, however the ad memcache module is not installed.

Here is the debug-2 data. I'd like to draw attention to the line:

PHP: errno(2048): Creating default object from empty value ; Line 21 in [/var/www/wikijob/modules/ad/imageserve.inc]

ad_dir: '/var/www/wikijob/modules/ad'
debug: '2'
adcache: 'none'
nids: ''
tids: ''
hostid: ''
url: ''
quantity: '1'
aid: '1016894'
ad_display: 'javascript'
--> Time mark: 14:27:42
Memory usage: 122.438 K
Drupal bootstrap '0'.
Override cache_inc: './modules/memcache/memcache.db.inc'
Drupal bootstrap complete.
PHP: errno(2048): Creating default object from empty value ; Line 21 in [/var/www/wikijob/modules/ad/imageserve.inc]
adserve_counter_image: increment 'view' counter for aid: 1016894
adserve_cache function(increment)
Cache function 'ad_cache_none_increment' does not exist. 
Memory usage: 138.141 K
Invoking cache function 'adserve_cache_increment'.
adserve_cache_increment action(view) aid(1016894) hostid()
Drupal bootstrap '8'.

Any help would be much appreciated!