Default route through MLVPN

[muesli]

                                               eth0 a.b.c.d
                                              +---------------+
                                 +----------->| Fast internet |--> IN/OUT
                                 |            +---------------+
                         mlvpn0  |
                  +--------------+-----+
          +------>| MLVPN Ubuntu       |<------+
          |       | server 11.12.13.14 |       |
          |       +--------------------+       |
          |            ^        ^              |
          |            |  T  A  |              |
 +--------+---------+  |  U  G  |  +---------+--------+
 | Link 1           |  |  N  G  |  | Link 2           |
 +------------------+  |  /  R  |  +------------------+
   192.168.10.2        |  T  E  |     192.168.11.2
          ^            |  A  G  |            ^
          |            |  P  A  |            |
          |            |     T  |            |
          |            |     E  |            |
internet 1|            |     D  |            | internet 2
          |            v mlvpn0 v            |
          |         +--+--------+---+        |
          +---------| MLVPN client  |--------+
                    | RaspPi        |
                    +---------------+
eth1.1: 192.168.10.1     mlvpn0  |     eth1.2: 192.168.11.1
                                 |
                                 | eth0: 192.168.0.1
 +-------------------+           |
 | LAN 192.168.0.0   |<----------+
 +-------------------+

This is my current setup and I'm struggling with MLVPN on multiple ends. To make things easier, I have eliminated the Link 2 entirely and am just trying to route everything through Link 1 for now.

I've followed the linux example from the docs and at the end, I am able to ping all my machines in the LAN from the MLVPN server. I can also reach the server from all LAN clients, but not from the RaspPi MLVPN client itself.

I seem to be able to get all my ICMP packages sent through the mlvpn connection and forwarded by the server, but as soon as I set up a default route on the MLVPN client all connections break. The TCP connects simply time out. I tried to lower the MTU, but that didn't seem to help. I don't think it's a NAT issue on the server either, that looks fine and works for regular openvpn connections.

Any ideas?

[muesli]

Ok, it looks like this is MTU related after all. If I reduce it to 1300 on all the machines in the LAN I can get a stable connection. Is there an issue with MLVPN and packet fragmentation?