I started having this problem recently as well. Posting was fine, but if anyone left a comment, they would be shown a blank page after clicking submit. I tried turning off each plugin individially, as well as upgrading WordPress.
While I was experimenting, I noticed that a visit who is not logged in would get the blank page, but if I - as admin - left a comment while logged in, I did not get the blank page. This led me to consider what might be triggered by an anonymous visitor that would not be triggered for admin. Further thinking led me to a solution. Since I have my system set to hold comments for Admin approval, an e-mail to Admin is generated when someone leaves a comment, but because comments by the Admin are auto-approved and not held for moderation, no e-mail is generated. It seemed likely that the e-mail was failing for some reason, and WP (or a plugin) didn't handle the error well enough to do anything but stop processing while trying to execute the remaining sections of "wp-comments-post.php". This was reinforced by the fact that I had not received any e-mails recently reminding me to moderate new comments.
Then I remembered that I had recently changed my Gmail password. The reason that that is relevant, in my case, is that I use the "Swift SMTP" plugin from Shift This to send e-mail from my site, which allows me to send mail through my Gmail account. I went into the Swift SMTP options, updated the password, and the blank page problem went away.
There may be some of you who do not use this plugin and are having this problem, but perhaps my experience will give you a hint of somewhere to look for a solution that you hadn't previously considered.