Bumping this as the most recent iteration of a long-standing problem; the diplomacy bug in particular has been reported a few times before that I've seen. 

You can still access diplomacy with individual empires by clicking on that empire's capital emblem, but that's a shoddy workaround at best, for several reasons. The main issue is that sometimes, especially in the mid-late game, you just can't find an enemy capital since it's buried deep in their wide-spread empire and they've closed their borders to you or else declared war, forcing you to slog through their many barricaded systems. Same problem can occur early game if you run into their scouts or probes a long way from their main system (or even main constellation). It also means that you can't see who's at war with or allied with whom, making diplomacy both more shallow and more difficult; and lastly it's just a lot more work to have to find a specific capital among several others than simply clicking on a button to meet them.

Official Bug Report for the Diplomacy Desync

Bug Summary: Clicking Diplomacy button on Mac OS causes the game to freeze.

Bug Desctiption: Upon clicking the Diplomacy button or attempting to use the hotkey on a Mac, the game turns unresponsive and the cursor becomes the Mac's signature Rainbow Wheel of Death. The cursor can still be moved but cannot interact with game in any way, nor can you use hotkeys. You can only quit the game by powering down computer or force-quitting from the dock. I've tried waiting out the freeze for up to thirty minutes, to no avail. I've tried it on both a MacBook Air and a Macintosh Desktop, both using the most recent OS version. Makes no difference at all.

Steps to Reproduce: 1) Start the game using the most recent Mac OS. 2) Encounter another major faction. 3) Press the Diplomacy Button or the hotkey.

Reproducibility: 5/5. Consistent as the sunrise.

Expected Behavior: Diplomacy Screen should open and display the various other factions you have encountered in the normal and appropriate fashion.