Reproduction steps:
1. Open the attached project.
2. Make sure Scripting Runtime Version is set to .NET 4.6.
3. Build and run it for iOS device.
- Application crashes:

frame #0: 0x017700ca ProductName`il2cpp::icalls::System::System::Net::Sockets::Socket::Connect(socket=0, socket_address=0x07a8c4c8, error=0x00000000) at Socket.cpp:485 [opt]
frame #1: 0x005fec38 ProductName`::Socket_SendTo_internal_m2511141814(__this=0x00000000, ___sock0=4, ___buffer1=0x07a8c4c8, ___offset2=0, ___count3=4, ___flags4=0, ___sa5=0x07a844e0, ___error6=0x4047a8d4, method=0x00000000) at Bulk_System_4.cpp:32707
frame #2: 0x005feb26 ProductName`::Socket_SendTo_internal_m2871471530(__this=0x00000000, ___safeHandle0=0x07a82d48, ___buffer1=0x07a8c4c8, ___offset2=0, ___count3=4, ___flags4=0, ___sa5=0x07a844e0, ___error6=0x4047a8d4, method=0x00000000) at Bulk_System_4.cpp:32673
frame #3: 0x005fd544 ProductName`::Socket_SendTo_nochecks_m2045631600(__this=0x07a81d20, ___buffer0=0x07a8c4c8, ___offset1=0, ___size2=4, ___flags3=0, ___remote_end4=0x07a8adf8, method=0x00000000) at Bulk_System_4.cpp:32594
frame #4: 0x005fdbfa ProductName`::Socket_BeginSendToCallback_m3713374952(__this=0x00000000, ___sockares0=0x08d20f08, ___sent_so_far1=0, method=0x00000000) at Bulk_System_4.cpp:32426
...

Reproduced with:
2017.1.0b1, 2017.2.0a2
.NET 4.6

Not reproducible with:
.NET 3.5

Note:
Full log attached in the edit.