Resolution

This happens because the output used by the I/O plug-in is silenced when Solo for the channel strip is engaged, interrupting the signal flow. Here is an example of how this can occur:

• You use Output 1-2 as the main output to your monitors.
• You have Output 3-4 of your audio interface connected to the inputs of an external piece of gear, such as a compressor.
• You have the output of the external compressor connected to Input 3-4 of your audio interface.
• You have a channel strip with an I/O plug-in configured like this:

When the audio signal for the channel strip reaches the I/O plug-in, it is sent to Output 3-4 and then returned to Input 3-4. When the channel strip is soloed, Output 3-4 is muted thereby interrupting the signal flow.

To allow solo with an I/O plug-in in projects that are set to manage channel strips automatically

If your project uses the default "Automatic Management of Channel Strip Objects" setting:

• In the Channel Strip Inspector area, click the output slot on the channel strip and choose the output pair the I/O plug-in uses. In this example it would be Output 3-4.
• The output channel strip to the right will change to Output 3-4. Control-click its solo button. A diagonal red line will appear across the button indicating it is now solo-safe.
  
• Put the channel strip output back to its original setting.

 If you would like to enable automatic channel strip management for your project:

• Choose File > Project Settings > Audio.
• Select the option for "Automatic Management of Channel Strip Objects"

To allow solo with an I/O plug-in in projects that are not set to manage channel strips automatically 

1. Choose Window > Environment.
2. Choose New > Channel Strip > Output.
   
3. If the channel strip is not set for the correct outputs, go to the channel strip parameter box at the left side of the screen, and set the Channel as needed.
   
4. To set the channel strip to Solo Safe, press the Control key on your keyboard and click its solo button.