I doubt that the type of server x86 or BananaPI has anything to do with it, i have a x86 server and i have a similar observations with a older Fritz!Box 6360 model. It does not crash, but internet connections will fail while syncing large amounts from inside the LAN.
If i use the dyndns domain in the seafile client the Fritz!Box has to route/forward all the packages from your public IP address to the Local IP address of your seafile server, rewriting each package in the process. When your client is also on the internal LAN these arrive at a speed it simply can not handle that fast.
As workaround for clients which aren't mobile like desktops pcs you can simply use the internal hostname or IP address of the server in the seafile client. (you may have to set the ignore ssl error option to do so)
Then communication of server and client will work directly without involving the poor little Fritz!Box and sync goes at full speed of server/client.