Cannot connect to RP via SSH any more

[Program]

After disconnecting the RP from the power supply, replacing the SD card and connecting the RP to the power supply again, I can no longer access the RP via SSH through putty for Windows. The RP shows up in the network with the correct IP address and I can access the web interface by entering its IP address in my browser. However, as soon as I try to open a connection with putty (using the same IP address and port 22), the putty command window opens and immediately closes again. I've tried several times and sometimes the window doesn't close immediately, but instead the message "Connection closed by remote host" appears and the putty window remains open, but says "inactive". Any ideas what the problem might be? I'm using RP version 1.1 with OS 0.95 (latest release as downloaded from the RP website). Connection via serial console works.
I'm not sure whether this is relevant, but before I unplugged the power supply from RP, I ran the commands "ro" and "killall nginx" in putty.