Notice: unserialize() error

I am getting an issue and have been trying to find a solution for ages - everthing that dicusses this issues seems to get no response.

If you could point me in the right direction... any assistance would be greatly appreciated.

Running Drupal core 5.3 with Taxonomy & CCK. Template is custom.

The returned error i am getting in the admin area when updating information and occasionally when navigating areas in the admin is:

Notice: unserialize() [function.unserialize]: Error at offset 2 of 42 bytes in /clientdata/www/au/a/arrivealive.vic.gov.au/includes/bootstrap.inc on line 390

Warning: Cannot modify header information - headers already sent by (output started at /clientdata/www/au/a/arrivealive.vic.gov.au/includes/bootstrap.inc:390) in /clientdata/www/au/a/arrivealive.vic.gov.au/includes/bootstrap.inc on line 531

Warning: Cannot modify header information - headers already sent by (output started at /clientdata/www/au/a/arrivealive.vic.gov.au/includes/bootstrap.inc:390) in /clientdata/www/au/a/arrivealive.vic.gov.au/includes/bootstrap.inc on line 532

Warning: Cannot modify header information - headers already sent by (output started at /clientdata/www/au/a/arrivealive.vic.gov.au/includes/bootstrap.inc:390) in /clientdata/www/au/a/arrivealive.vic.gov.au/includes/bootstrap.inc on line 533

Warning: Cannot modify header information - headers already sent by (output started at /clientdata/www/au/a/arrivealive.vic.gov.au/includes/bootstrap.inc:390) in /clientdata/www/au/a/arrivealive.vic.gov.au/includes/bootstrap.inc on line 534

Thanks in advance