There is a bug in the new Minecraft horse update that makes the launcher incompatible with OSX 10.5.8. This only happens when logging in to play. The system doesn't continue further. The launcher load but once you click play it doesn't go any further.
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fuj1n
Works as intended, please create a new profile, and set the version to 1.5.2 to continue.
fuj1n
Please do not mark issues as private, unless your bug report is a security issue containing information that may compromise your server/client.