I have a RB751n sitting around which was running 5.25 (or similar). I updated it to 6.5 by uploading the mipsbe .npk over the web interface.
It took a long time (5+ minutes to update). After 10 minutes, I got impatient and pulled the plug on it. When it rebooted, it took a few minutes to start working again.
Now it is running (NAT, DHCP, Bridge etc) but.. Winbox is VERY slow to log in, and if I can log in, doesn't show data (interface list is blank). SSH times out. HTTP web page times out.
The router seems to be functioning normally otherwise, and Winbox reports it is running 6.5. But it's obviously not right - what did I do wrong.. and how can I fix it?!