Issues which got fixed in Beta 3 are tagged as [Fixed B3] in this forum.
Most important fixes:
fixed a crash in Renois, that could happen after menu entries got dynamically removed by tools (via renoise.tool():remove_menu_entry)
OSX: fixed delayed, missed OSC messages (UDP packages in general - to be exact) - could maybe also happen on Linux
empty lists in tool preferences did not properly save/restore
UDP servers (from renoise.Socket) will no longer pass the connected socket with received messages, but only a socket which can be used to query the messages peer port and address.
renoise.tool().new_document_observable is now also fired when a tool reloads (either because of other tool installations or when AUTO_RELOAD_DEBUGGING or when using the “Reload All Tools…” option in Renoise)
renoise.song().instruments.samples.sample_buffer.selection_start, selection_end and selection_range are read-write now (but not yet observable)
Fixing bugs has the highest priority now. With the beta getting more stable, we hopefully will have time for other API additions/changes in future 2.6 Beta releases.