The checksum is wrong on the receiving end. Something has smashed some packets on the way. Changing the MTU was a good idea. Too bad it didn't work. Is Path MTU Discovery (PMTUD) broken somehow? Are you dropping ICMP at your router? For PMTUD to work you need to at least accept ICMP Unreachable (type 3) messages. The Don't Fragment (DF) bit is set, so if the MTU is exceeded at any hop along the path you will get the problem you are seeing.
My guess as to why ssh works but not scp is ssh typically never sends full datagrams.
