I can't believe that this is caused by server side code. The browser doesn't see that. So be careful not to rush to judgement when you say "This seems to be an issue with the server-side code, as the client side code is working just fine."
Historically XPages apps have broken when a browser goes incompatible with the dojo controls. this has happened at least twice that I recall.
First think I suggest you do is upgrade to the latest FixPack. #9 just came out. FP4 came out in July of 2015.