This issue is caused by corrupted metadata or cross-mapped control data introduced during the Logic Pro 12.3 update. Since brand-new projects work perfectly, your plugins and audio engine are healthy—the issue is isolated to how updated Logic reads legacy project containers.
Here is the quick fix checklist:
1. Toggle Core Audio (Reset Mixer Engine)
Forces Logic to completely rebuild the audio routing and re-initialize frozen plugin instances.
Go to Logic Pro > Settings > Audio > Devices.
Uncheck Core Audio Enabled, click Apply Changes.
Re-check Core Audio Enabled, click Apply Changes.
2. Bypass Control Surfaces (Fixes Random Soloing)
Corrupted controller assignments are likely hijacking your plugin bypass states and cross-mapping the Solo buttons.
Go to Logic Pro > Control Surfaces > Preferences.
Check Bypass All Control Surfaces.
If this fixes it, go to Logic Pro > Control Surfaces > Controller Assignments, switch to Expert view, press Command + A, and Delete the corrupted maps.
3. The "All Files" Import Workaround
If the steps above fail, the legacy .logicx file container itself is corrupted. You can easily pull the uncorrupted data into a healthy environment:
1. Create a brand-new, empty project.
2. Open the Browser pane on the right (F) and select the All Files tab.
3. Locate and double-click your corrupted project file to expose its internal tracks.
4. Check the boxes for Content, Plugins, Sends, and I/O for all tracks.
5. Click Import at the bottom of the browser.
Tip: Always duplicate your original project file (Command + D in Finder) before troubleshooting
