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Dual display setup stopped working after Device support update

I have two external displays connected to my MacBook Pro 16" (2019), running Big Sur 11.16.1. Both displays are a Philips PHL 279C9. So far, I always had two working displays in addition to the MacBook built-in display.


Last Friday I did the MacOS Device support update at the end of the day, after which I shut down my MacBook. On Monday I booted and only to notice that just one of the external displays responds. Switched cables back and forth a couple of times, but that only resulted in the other display to not respond. Can't get it back to work with both external displays.


How can I solve this?

Posted on Nov 9, 2021 5:09 AM

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8 replies

Nov 11, 2021 3:56 AM in response to Luis Sequeira1

Thank you for your quick reply, much appreciated.


I'm using the simplest setup possible I guess: two USB-C ports connect both displays, so no hub or whatsoever in between.

After I encountered the issue, I did switch around the cables to the ports and displays to rule out malfunction of both the ports and displays. All are working fine.


Please find the Etrecheck report attached.





Nov 11, 2021 3:17 AM in response to Kapitein Mulshoop

I don't see how you can downgrade unless you can restore from a Time Machine backup prior to the point at which you installed this.


I am on Monterey, with a similar mac (2019 16" MBP) and while I don't have identical monitors like you, I can use two extra monitors (well, so far I've only had the chance to test with an external display plus an iPad using Sidecar, but that is still two external displays).


How are the two displays connected?

While maybe unlikely, there is a chance that some other software you have may be causing a conflict.

Please run Etrecheck and post its full report here. Use the "additional text" button and paste the report into the text box.

We may see something in the report that can help us figure this out.


Nov 11, 2021 4:53 AM in response to Kapitein Mulshoop

The report shows some serious issues in your setup. They may or may not be directly affecting the problem in this thread, but they should be taken very seriously and addressed urgently.


Major Issues:

Anything that appears on this list needs immediate attention.

Proxies - Network proxies detected. This could be evidence of malware.

Unsigned files - There are unsigned software files installed that could be adware and should be reviewed.

Automatic updates disabled - Automatic updates are disabled. This computer is at risk of malware infection.

Security updates disabled - Security updates are disabled. This computer is at risk of malware infection.

Apple security disabled - Apple security software is disabled. This computer is at risk of malware infection.



Network proxies: typically, there should be none. Do you have any reason to have proxies?

They are usually installed by malware and can redirect your internet traffic in malicious ways.

Go to System Preferences->Network, select the service you use (Wifi, Ethernet...), click Advanced... and then click Proxies.

What do you see?


System security is disabled - why?????


And in turn you have installed tons of configuration profiles. This is unusual.


Is this a mac run by an IT department or a personal mac?

If the former, you have to ask for their help.

If the latter, you have installed lots of useless and even damaging stuff from macafee, while turning off the system's own security.

I have to say that this looks like a very poorly managed security situation.


The immediate remedial steps are to turn system security back on, and completely uninstall mcafee.

Nov 11, 2021 6:09 AM in response to Luis Sequeira1

Thanks again Luis,


This MacBook is indeed run by an IT department. Sorry, should have pointed that out. That's the reason multiple profiles and network proxies are installed.

McAffee is useless indeed, but hey, company policy 🤷‍♂️


I've asked for help of the IT department. They can't help me they say, because I'm working from home. Bringing two external displays to work so they can have a look, is obviously not something I'm going to do.

Nov 11, 2021 8:57 AM in response to Kapitein Mulshoop

I am not sure what else we can do.

The company policy requires installing crappy antivirus, but disabling the built-in system security does not seem to be a problem for them...


Maybe you can take the laptop and test at the office with some other displays?


If you could install a clean system on a separate drive, you would be able to confirm that it worked fine with the two displays. I don't know if you can do that, of course, but it would prove that some third party stuff - possibly mcafee or one or more of the profiles installed - was blocking the use of both displays.

Nov 11, 2021 10:52 AM in response to Luis Sequeira1

Unfortunately I'm not allowed to do clean system installs and such. It's company managed, so this is prohibited.


Thanks for the tip about taking it to the office to see if it works with two other displays. Didn't think of that.


Besides that I guess I have to hope that the installation of Monterey - which should soon arrive company wide - will solve this. Thanks for your help Luis.

Dual display setup stopped working after Device support update

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