I can't write comments.
There was no issue with the comment system. Instead, there was a bug on the bounty system, and the error message "Unable to load popup - please try again" appeared when "start a bounty" was clicked.
The bug should have been fixed, as per Eeeek! Can't start a bounty on any question on Meta SE.
Thanks for reporting this. It has been fixed across the network now and you should be able to add bounties until your heart is content.
Thanks to balpha who smashed the bug very quickly.