The fact that “JUCE 6.1.6” has been printed twice implies that the error is coming from a plugin that’s been loaded (the first JUCE is probably from the AudioPluginHost, the second is from somewhere else, likely a plugin). If you’re testing your own plugin, then perhaps you’ve introduced a bug somehow.
Things to try:
Look at the stack trace at the point of the error. This should clearly indicate where the error is coming from.
Test your project in standalone instead of as a plugin. Do you still see the same error?
Delete the AudioPluginHost settings file from ~/Library/Preferences/Juce Audio Plugin Host.settings and try re-testing. After deleting the settings file, the host will load in a default (blank) state, so if you still see the assertion you’ll know it’s an issue in the host itself - otherwise, you’ll know the problem is in one of the plugins being hosted.