The following errors occur on a Draytek IPsec if the VPN name has been modified:

ISAKMP Identification Payload has an unknown value ...
...
ISAKMP payload must be zero
...
... smalformed payload: probable authentication (preshared secret) failure
...
Matching General Setup key for dynamic ip client ...

To fix the problem it is necessary to clear the VPN configuration that was renamed and re-add it.