Network loopback device missing in Ubuntu 14.04

I want to program an FPGA via JTAG and the xmd script connects to 127.0.0.1:3121 to do so. That worked nicely for months. After a reboot this connection suddenly didn't exist any more, I cannot ping 127.0.0.1 or localhost (still resolved to 127.0.0.1) any more either. ifconfig is missing "lo". I have entries for eth4 and eth5, my two network cards, but no lo. Full output is at the end of the question. Connections to and from other computers work fine. How do I get it running again?

I tried to start it manually but that doesn't change anything:

> ifup lo
ifup: interface lo already configured

> ifup --force lo  
(no output)

This doesn't work either:

> route add -host 127.0.0.1 dev lo  
SIOCADDRT: No such device

Some diagnosis output:

> route -n  
Kernel IP routing table  
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface  
0.0.0.0         192.168.153.1   0.0.0.0         UG    0      0        0 eth5  
192.168.10.0    0.0.0.0         255.255.255.0   U     1      0        0 eth4  
192.168.153.0   0.0.0.0         255.255.255.0   U     1      0        0 eth5  

> cat /var/run/network/ifstate  
lo=lo

> cat /etc/network/interfaces  
# interfaces(5) file used by ifup(8) and ifdown(8)  
auto lo  
iface lo inet loopback

> iptables -L  
Chain INPUT (policy ACCEPT)  
target     prot opt source               destination           

Chain FORWARD (policy ACCEPT)  
target     prot opt source               destination           

Chain OUTPUT (policy ACCEPT)  
target     prot opt source               destination

I don't have an /etc/iptables.conf

I also tried sudo rm -rf /var/run/network/* from this question. I did apt-get update and apt-get upgrade. No change.

> ifconfig 127.0.0.1 lo  
lo: Unknown host  
ifconfig: `--help' gives usage information.

Output of ifconfig:

eth4      Link encap:Ethernet  HWaddr 00:24:e8:45:8f:10    
          inet addr:192.168.10.2  Bcast:192.168.10.255  Mask:255.255.255.0  
          inet6 addr: fe80::224:e8ff:fe45:8f10/64 Scope:Link  
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1  
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0  
          TX packets:67 errors:0 dropped:0 overruns:0 carrier:0  
          collisions:0 txqueuelen:1000   
          RX bytes:0 (0.0 B)  TX bytes:10916 (10.9 KB)  
          Interrupt:21 Memory:f7fe0000-f8000000   

eth5      Link encap:Ethernet  HWaddr d8:eb:97:71:be:e5    
          inet addr:192.168.153.124  Bcast:192.168.153.255  Mask:255.255.255.0  
          inet6 addr: fe80::daeb:97ff:fe71:bee5/64 Scope:Link  
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1  
          RX packets:35392 errors:0 dropped:0 overruns:0 frame:0  
          TX packets:28754 errors:0 dropped:0 overruns:0 carrier:0  
          collisions:0 txqueuelen:1000   
          RX bytes:20734790 (20.7 MB)  TX bytes:4354467 (4.3 MB) 

Edit:

> ip link  
1: ttyUSB0: <LOOPBACK> mtu 65536 qdisc noop state DOWN mode DEFAULT group default  
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00  
2: eth5: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000  
link/ether d8:eb:97:71:be:e5 brd ff:ff:ff:ff:ff:ff  
3: eth4: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000  
link/ether 00:24:e8:45:8f:10 brd ff:ff:ff:ff:ff:ff