'Notice: unserialize() [function.unserialize]: Error at Offset' when changing event dates

On a live server (problem never occured on development server) all events created using Events module have reset to 1 Jan 1970 and any attempt to change them fails and gives this error:

Notice: unserialize() [function.unserialize]: Error at offset 1016 of 1016 bytes in /home/fhlinux166/w/womeninjournalism.co.uk/user/htdocs/includes/bootstrap.inc on line 390

Having tried updating Events to dev version and then changing it back to same version on dev server, I am convinced this is a server / Drupal issue rather than the module itself.

Does anyone know what is causing this? Googling the problem gives a wide variety of reasons but nothing I can pinpoint. I'm not a PHP developer so this isn't really my strong point....

Thanks

R