I also updated to AE 6.5.1 recently and my final render works fine.
Im running OS 10.4.2 and AE 6.5.1. pro.
I just did a render on a 20 Layer project that’s 15 sec long.
The only thing that i can say is that it took a minute or two longer than before the upgrade.
There is Under your preview window a resolution control you have to set it on FULL to see your comp in best quality.
The same thing when you are making a movie you have to set RENDER SETTING on BEST QUALIT and FULL RESOLUTION.
When your making a RAM PREVIEW by pressing 0 (zero) you have to set the resolution on FULL in TIME CONTROL window.