My testing process:
I run a server with a few custom Add-ons.
After the 1.26.30 update, players would get stuck on "Connecting to the server".
I manually updated all my packs manifest files, bumping the min_engine_version to 1.26.30 and migrating the scripts from the beta API to the stable API.
Despite the code being fully updated and stable, the server still refused player connections.
The fix: The only way to let players join again was to completely disable the Add-ons and replace the world's level.dat with a Vanilla one (with all Experimental toggles turned to false/0).
The connection hang/timeout specifically happens on worlds that have Experimental Features enabled in their level.dat (such as Beta APIs, Holiday Creator Features, or Upcoming Creator Features).
I am experiencing this exact same issue on BDS 1.26.30. After doing some extensive testing with my community's server, I've managed to narrow down the root cause.
Conclusion: The 1.26.30 BDS update seems to have completely broken the handshake or world-loading process for any server trying to load a level.dat that has creator experiments toggled on.
Temporary Workaround for other Admins: If you want your players to be able to join right now, you must disable your experimental Add-ons and turn off the experimental toggles in your level.dat (playing purely Vanilla) until Mojang releases a hotfix.