Changing the fragmentation policy will affect anti-replay window.

The concentrator you can change the Fragmentation Policy under:

Configuration | Interfaces | Public Interface | Public Interface IPSec Fragmentation Policy

And set it to:

Fragment prior to IPSec encapsulation without Path MTU Discovery (Clear DF bit)?

You can also lower down the MTU to 1300 on the same window.

Try these links:

http://www.cisco.com/univercd/cc/td/doc/product/vpn/client/3_6/admin_gd/vcach1.htm#wp1153282

http://www.cisco.com/en/US/products/sw/iosswrel/ps5207/products_feature_guide09186a00802299fb.html#wp1027258