MacOS – OS X Dock wont let me remove icons

Suddenly since yesterday my dock contains 3 apps/folder that I cannot remove:

• App Store (represented with ? icon)
• System Preferences (represented with ? icon)
• Applications folder

Neither lets me remove it from the dock.

I was using High-Sierra and now Catalina – same thing. I've tried killall Dock and killall Finder but nothing changed, I've deleted the dock.plist and restarted but nothing happened as well.

How can I solve this? Thanks.

P.S. Regarding the two apps, both "normal" versions can be added and removed from the dock, I don't know where this two "ghosts" came from. Same for all folders (excluding Applications).

EDIT 1: 29-11-2019 10:30 UTC:
I thought I've solved it but no.
I made a search by com.apple.dock and found another .plist at /Library/Managed Preferences/[user]/com.apple.dock.plist:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>MCXDockSpecialFolders</key>
    <array/>
    <key>contents-immutable</key>
    <false/>
    <key>static-apps</key>
    <array>
        <dict>
            <key>mcx_typehint</key>
            <integer>1</integer>
            <key>tile-data</key>
            <dict>
                <key>file-data</key>
                <dict>
                    <key>_CFURLString</key>
                    <string>/Applications/System Preferences.app</string>
                    <key>_CFURLStringType</key>
                    <integer>0</integer>
                </dict>
                <key>file-label</key>
                <string>System Preferences</string>
            </dict>
            <key>tile-type</key>
            <string>file-tile</string>
        </dict>
        <dict>
            <key>mcx_typehint</key>
            <integer>1</integer>
            <key>tile-data</key>
            <dict>
                <key>file-data</key>
                <dict>
                    <key>_CFURLString</key>
                    <string>/Applications/App Store.app</string>
                    <key>_CFURLStringType</key>
                    <integer>0</integer>
                </dict>
                <key>file-label</key>
                <string>App Store</string>
            </dict>
            <key>tile-type</key>
            <string>file-tile</string>
        </dict>
    </array>
    <key>static-only</key>
    <false/>
    <key>static-others</key>
    <array>
        <dict>
            <key>mcx_typehint</key>
            <integer>2</integer>
            <key>tile-data</key>
            <dict>
                <key>file-data</key>
                <dict>
                    <key>_CFURLString</key>
                    <string>/Applications</string>
                    <key>_CFURLStringType</key>
                    <integer>0</integer>
                </dict>
                <key>file-label</key>
                <string>Applications</string>
                <key>file-type</key>
                <integer>2</integer>
            </dict>
            <key>tile-type</key>
            <string>directory-tile</string>
        </dict>
    </array>
</dict>
</plist>

Deleted this bastard, killall Dock, and back to normal. The good one is at /Users/[user]/Library/Preferences/com.apple.dock.plist

But today, when I booted my Mac this icons came again… And the procedure I follow yesterday doesn't work anymore. Removing the "bad" .plist or replace it with the good one, does nothing. Even after a reboot or killall Dock 🙁

EDIT 2: 29-11-2019 15:37 UTC:
After performing the actions suggested by @NathanStrik nothing happened except that this time, after deleting the "bad" .plist and killall Dock the icons disappeared. This is so confusing…