Thanks for your report. Does PuTTY have the same problem on renegotiation?
Windows Vista, WinSCP 5.5.6, server OpenSSH_6.4p1, OpenSSL 1.0.1e-fips 11 Feb 2013
Copying a directory from server to client.
Simultaneously using putty to same server for entering a few commands.
Working fine over a high signal WLAN connection until key renegotiation.
Then it just stopped, displaying no data transferred and incrementing estimated time left.
Clicking cancel followed by yes/no/cancel did nothing. Neither clicking on the closing X on the windows.
Had to kill the processes using the task manager.
< 2014-12-08 21:03:36.820 C0755 482543104 0000F621.DAT
. 2014-12-08 21:03:36.952 File: '/data/Agent7/Data' [2011-01-14T23:25:36.000Z] 
. 2014-12-08 21:03:36.974 Sending NULL.
. 2014-12-08 21:03:36.975 Binary transfer mode selected.
. 2014-12-08 21:03:56.496 Initiating key re-exchange (too much data received)
. 2014-12-08 21:03:56.687 Doing Diffie-Hellman group exchange
. 2014-12-08 21:03:56.719 Doing Diffie-Hellman key exchange with hash SHA-256
. 2014-12-08 21:03:58.219 Initialised AES-256 SDCTR client->server encryption
. 2014-12-08 21:03:58.264 Incoming packet was garbled on decryption
* 2014-12-08 21:03:58.349 (ESshFatal) Incoming packet was garbled on decryption
(end of log)