cant run diagnotic mode - error 2101

Suggest downloading the Application Etrecheck directly from a well Respected ASC Contributor. 

Disclaimer: I am not affiliated with nor receive compensation from the Developer.

The application is free or paid from added features. 

Run the application with Full Disc Access ( Security & Privacy - Full Disc Access ).

It will take a Snap Shot -  both the hardware and software.

 The Report will Not Reveal Any Personal Information. 

Post back the Full Report - copy and paste - using the Additional Text Icon ( 3rd Icon to last )

We can have a look at the report for possible issues and may have possible suggestions to resolve the issues.

Any Third Party Applications that will interfere with the normal operation of the OS, alter, modify, remove or delete or attempt to do so is an invitation for disaster and may require a Reinstallation of the OS.

This includes AntiVirus, Disk Cleaners, Disk Optimizes, UnInstaller etc.

The The Built in Security  is all that is required.

I don't think so because I ran it in customize mode on a 2011 iMac and didn't experience overheating. But that was an iMac vs your MBP.

I'd fully reinstate the SIP as follows:

1 - Boot into the Recovery volume (boot with the Command + R keys held down).

2 - launch the Terminal.

3 - enter the following command followed by the Return or Enter key:

csrutil enable

Then download and run Etrecheck.  Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the Etrecheck's Privacy preference pane so that it can get additional information from the Console and log files for the report:

Also click and read the About info to further permit full disk access.

Copy the report

and use the Additional Text button to include the report in your reply.

Ethernet Cable to Router of Wifi connection ?

If Ethernet - just abort for now and they agin at a later date - time

Apple Servers seem too be having issue for  How to get old versions of macOS  too since Friday and from I can see still having issues today Saturday.

The two might be connecting in non-functioning.

Thank you for the Report.

The two biggest issue as I see it is AdGuard and the Custom Kernel Boot Arguments

That and the disabling of some key Apple Protections available as part of a normal Installation of Big Sur

Antivirus software: AdGuard

A - There are no known Windows-like Viruses in the wild that self replicate and affect macOS, because of the underling UNIX  Foundation and Permission Limitation. 

Details: Restrictive config permissions - possibly adware

System Integrity Protection: unknown (Custom Configuration)

Kernel boot arguments - This computer has custom kernel boot arguments.

Kernel boot arguments - This computer has custom kernel boot arguments.

System modifications - There are a large number of system modifications running in the background.

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

System Integrity Protection disabled - System Integrity Protection is 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.

Clean up - There are orphan files that could be removed.

Kernel extensions present - This computer has kernel extensions that may not work in the future.

I believe the SIP info is stored in the NVRAM so do the following: Reset NVRAM or PRAM on your Mac - Apple Support and try reenabling the SIP again (check to see what the Terminal says before booting into Recovery mode).

If not mistaken - the error appearing is alerting the user either there is no active and functioning Internet Connection or the Apple Servers are too busy or not working.

Enter the following command in Terminal followed by the Return or Enter key:

csrutil status

Let us know what it tells you

It depends on why it was customized. I ran a customize SIP a couple of systems bak so I could run a system dock modification (make the dock transparent) but it was no big problem. It still protected but allowed the mod.

The csrutil status command will tell benjigorden if it's enabled or not. If' it's enabled and he doesn't know why we can give him the commands to reenable it.

They were yesterday and some unconfirmed report still today