Hi,
Have a look at the isakmp policy on the router, which should be configured something like this....
Router example:
crypto isakmp policy 10
encr aes 128
group 5
On the ASA isakmp is probably referred to as IKEv1 (depending on your version), something like this....
ASA example:
crypto ikev1 policy 10
encryption aes-128
group 5
The policy number is not so important, it will process/prioritise the lowest number first.
The algorithms need to match, the policy number does not. Make amendments if required, if still an issue please run debugs and upload the output "debug crypto isakmp|ikev1"
HTH