Why does SCP hang on copying files larger than 1405 bytes?

This definitely sounds like MTU problems (like @Konerak pointed out), this is how I would test this:

ip link set eth0 mtu 1400

This temporarily sets the allowed size for network packets to 1400 on the network interface eth0 (you might need to adjust the name). Your system will then split all packets above this size before sending it on to the network. If this fixes the scp command, you need to find the problem within the network or make this ugly fix permanent ;)

Consider that the MTU is typically 1500 B (due to the limitations of ethernet). Of those 1500 B not all are used for 'data'. So-called protocol overheads take a hefty chunk out of the 1500.

• SCP (uncompressed) requires a 65 B header.
• TCP requires a 20-60 B header.
• IP requires a 20-60 B header.

In light of this, it's not so surprising that your payload is limited to 1405 B.

P.s. Have a play with wireshark and check the IP headers. Are they allowing packet fragmentation?