XF 1.4 Can't reply to one particular PM

Hi, I'm having an issue with my Xenforo board I just migrated over from myBB 1.6. Absolutely loving the software, but I'm just having one niggling issue. I have a PM from a user, which I need to reply to, but I can't. It isn't because it's locked or anything, the reply box and button are still there. When I click reply, the box greys out as if it's trying, then just goes back to how it was, with my message still in the reply box. Any ideas? All other PMs seem to be fine.

Thanks :)
Disabled the ad blocker, fixed the issue. Turns out my whitelist got refreshed when my browser crashed a couple of days ago. Oh well, thanks for the pointer :)