Problem: "Connecting..." for some time and then the message "Connection lost".
Fix: Removing old keys and signatures from all mods.
Specifically I searched my mod folder for mods that had .bikey files for old versions and then ran the following to remove all keys and signatures.
del /s *.bikey
del /s *.bisign
Afterwards I simply repaired the mod via the Arma 3 Launcher to add the correct signatures.