Still doesn't work...
As a follow-up things have now changed, situation is now:
'assign to' function was working fine on previous server running windows 2003 with the following app versions:
Apache 2.0.63
PHP 5.2.1
mysql 5.0.51
phpmyadmin 2.11.51
we can close tickets normally when un-modified osticket files are in use, when we install the 'assign to' mod, closing ticket is no longer possible (error message as previously described)
On the new 2008R2 server apps are as follows:
Apache 2.2.21
PHP 5.3.9
mysql 5.5
phpmyadmin 3.2.0.1
we do still see some warnings about deprecated functions when closing the ticket: "Assigning the return value of new by reference is deprecated in D:\vhosts\osticket\include\pear\Mail\mimeDecode.php on line 335"