First and foremost check to make sure your system can run 10.11. Look up your Mac here: https://support.apple.com/en_US/specs/macmini
NOTE: Each new Mac OS X version that comes out requires / run better on more memory. Consider getting a memory upgrade to 8GB or more to run effectively.
Studio One V1 is not supported on 10.11. If you wish to continue to use it, you may do so at your own risk. We have found that Studio One v1 will exhibit erroneous behavior. We recommend upgrading to Studio One 3 for use with 10.11.
As for your 24.4.2 mixer. You will need to completely uninstall any previous version of Universal Control you have installed on your system. Apple changed their OS drastically in 10.11 removing core functions which disables older softwares entirely, not just our own.
Follow this tutorial on how to remove the files from your system:
Then install UC 1.7.3 found on our download page here: http://www.presonus.com/support/downloads/StudioLive-24.4.2
NOTE: installing UC 1.7.3 and not removing older versions will not work. The software removal process must be done completely before installing 1.7.3 in order for this to work.
The best thing to do is to partition your drive so that you can install 10.11 on a separate partition and reinstall your software one at a time. We've been finding customers who have upgraded from older versions of Mac OS X prior to 10.10 have problems with software not working or blocking functionality.You'll save yourself a ton of time by doing it this way and then using your older OS X install as a 2ndary data drive.