Some Ajax calls never return, eventually timeout with a 502 proxy error

The "post", "reply", "delete", "save as draft" buttons as well as the autosave that generate Ajax calls to our server are never returned, leaving the code comment dialog box unresolved (buttons are shown greyed for five minutes, followed by the "error communicating with server" dialog). The actions are performed correctly as refreshing the page will show the posted comment, etc.

The attached log [see private comment] is from Crucible with debug and network logging enabled for five minutes starting with the start of a deleteCommentAjax POST (and repository Pinger activity grepped out). No corresponding end request appears in the log.