XF 1.4 "The server responded with an error."

I am guessing you are using Chrome?

http://stackoverflow.com/questions/1820927/request-monitoring-in-chrome
http://stackoverflow.com/questions/4939172/in-chrome-how-can-i-see-what-ajax-requests-are-being-made

Check if _xfResponseType=json is included anywhere in "General" or "Query Request Parameters".
I have once seen this issue and it was (I think) was because of a js conflict with some add-on or style and prevented XenForo to include the above mentioned request parameter. And funnily only on certain URLs :P
 
I actually don't remember :P
Just wanted to verify if this is the same as the one I encountered.

Hopefully someone will be able to help you. :)

Another quick question: are you using any custom style? If so, which one? And any add-ons related to threads and posts?
 
Hit "F12" for windows (or) cmd+opt+i for mac on keyboard when you get the error, then click on "Console" (check the screenshot below)
upload_2015-5-29_9-15-0.webp
and then screenshot it and upload it to this post.
 
nuKDDuY.png

And the red text continues..
 
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