[strongSwan] Packets get lost in one direction (MTU?)

[Catscrash]

Hi,

I have an issue with one of my tunnels. On both side installed is 
strongswan 5.5.1-4+deb9u4 on Debian 9.

Here is the config

conn connection1
         type=tunnel
         left=IP_Server_A
         leftsubnet=10.155.0.1/32
         leftfirewall=yes
         leftid=IP_Server_A
         right=IP_Server_B
         rightsubnet=10.100.0.1/24
         rightid=IP_Server_B
         auto=start
         compress=yes
         #Phase-1
         keyexchange=ikev2
         authby=secret
         ike=aes256-sha256-modp4096
         ikelifetime=24h
         #Phase-2
         keylife=1h
         esp=aes256-sha256-modp4096

Other side looks like that, with left and right switched.

Ping works from A to B and from B to A.
When I ssh from B to A, it works, but as soon as I have a larger 
terminal output the connection breaks.
When I ssh from A to B, everything works fine.

When I do a scp on server B to push a file to server A, everything works 
fine, even for huge files.

When I do a scp on server B to pull a file from server A, it breaks 
after a few bytes and doesn't continue.

I thought this sounds like a MTU issue. I tried setting 
fragmentation=yes, which did not help. The external interfaces on both 
sides have mtu 1500 set, at least that's what "ip link show" says.

I tried to find out which MTU would work with the ping -M do -s command. 
And it seems everything above 1410 causes trouble. So I guess setting it 
to 1400 would be fine... But how? I'd rather not set the external 
interface MTU, since there are a lot of other tunnels on those servers 
that work just fine except for this one

Thanks for any help!||||
||

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