L2TP client-to-site VPN "asynchronous network error"

Hi

Ive set up an client-to-site remote access over L2TP on UTM 9.

UTM 9 is the server - a Synology Diskstation is the client.

Connection works, but I get very poor data rates (~200kb/s / 3MB/s possible). The log shows several errors like this:

ERROR: asynchronous network error report on eth1 for message to 95.222.24.192 port 29364, complainant 95.222.24.192: No route to host [errno 113, origin ICMP type 11 code 1 (not authenticated)]

What does this mean and how can I correct this?

PPTP does work like a charm (with full speed), but I am trying to avoid PPTP because it is considered unsecure.

Site-to-site does not work unfortunatley, because the client is behind a NATed IPv4 (DS-lite).

Thanks for you help,

Frank