A Reddit user found a solution: https://old.reddit.com/r/MacOSBeta/comments/1fhl39c/system_extensions_issue_with_latest_beta/lo4v4hc/ https://old.reddit.com/r/M

A Reddit user found a solution: https://old.reddit.com/r/MacOSBeta/comments/1fhl39c/system_extensions_issue_with_latest_beta/lo4v4hc/ https://old.reddit.com/r/MacOSBeta/comments/1fhl39c/system_e... I've noticed he disabled "Find My Mac" in the first step and only enabled it again after everything was done. This seems to be the key. I suspect the "SDErrorDomain error 104" is related to the "Activation Lock" aka. "Find My Mac". I've disabled Find My Mac, booted into recovery and was finally able to set it to "Full Security" and back to "Reduced Security" again without problems. After that and a fresh boot, the popup message about the updated extension was gone. Enabled "Find My Mac" again (after setting the system to "Full Security" first - as that's the setting I intended to use) and all is well. nb. SIP was enabled for me the whole time. It was the security mode running on "Reduced Security" (both checkboxes checked) which was probably the cause for the "updated extension" issue.