SSH session through OpenVPN cuts off / locks up after a few lines

You have the symptoms of an MTU problem: some TCP connections freeze, more or less reproducibly for a given command or URL but with no easily discernible overall pattern. A telltale symptom is that interactive ssh sessions work well as long as you don't run commands with large output. See Can't access select https sites on Linux over PPPoE for an explanation.

OpenVPN has several MTU-related option — search for “mtu” in the manual. I don't have enough experience to be confident as to which option you need to change. (It's even possible that you can change something in the Wimax modem configuration.) The most likely option to change is mssfix: try lowering the value until it fixes the problem. The default is 1450; something like around 1400 might fix your problem. Try openvpn --fragment 1200 -mssfix; if it helps, increase the value until it starts breaking.

Gilles' answer is completely correct, but there is also another potential cause for this.

There was a bug in version 2.3.0 of OpenVPN which would disconnect clients when sending large chunks of data: https://community.openvpn.net/openvpn/ticket/263

This issue only occurred when using TCP. UDP was completely unaffected.

We had the same problem, and it was indeed a MTU issue. However for us the problem was not in openVPN configuration but on the tun0 interface.

How we solved it: first find the maximum packet size that went through, with

ping <host> -s 1500 -M do

and reducing the 1500 value until a value was going through (for us, 1350).

Once the right value was found, change the tun0 interface with

sudo ip link set dev tun0 mtu 1350

as was proposed by Sebastian here. After that the VPN was going smoothly.