Account sometimes partially lose administrator status

Windows 10 1903 x64

For a few months now, sometimes, my account (Local Administrator) will partially lose its administrator privileges, making things happen such as:

• task manager, when launched normally, doesn’t allow killing system processes
• device manager shows a warning telling me I don’t have the required privileges
• gpedit.msc shows an error telling me I am not allowed to edit the group policies for this computer
• services.msc does the same
• all .msi files become nonworking, give error 2755 after clicking on "Install" (Note: this is not a Windows Installer problem, it’s only part of the bigger problem I’m describing here)
• can’t edit system environment variables if I open the window from the control panel (the buttons are greyed out)
• can’t edit system registry keys from the registry editor

I can, however, perform any of these tasks, by launching an elevated command prompt (Win+X, A), and typing the corresponding commands.

My first guess was a misconfigured UAC, but it’s at the default setting and I tried other settings, to no effect. I tried clearing the GPOs for the computer, the problem persists.

The problem usually happens after a few hours after logging in, i.e. I can start the task manager right after logging in and I’ll get the normal privileges and view, but if I close it and re-start it a few hours later, in the Details tab the "User" column will be blank for processes not belonging to my user, and I’ll be unable to perform any task except from killing my own processes.

Logging out and back in solves the problem most of the time, but sometimes a reboot is required.