"perl/Tk unusable, cannot create main windows." when running using "sudo"

Full error message for sudo /usr/local/bin/tlmgr -gui (TeX Live 2017 manager) I'm getting on my OpenSUSE Tumbleweed with Gnome 3:

No protocol specified perl/Tk unusable, cannot create main windows. That could be a consequence of not having X Windows installed or started! Error message from creating MainWindow: couldn't connect to display ":0" at /usr/lib/perl5/vendor_perl/5.24.1/x86_64-linux-thread-multi/Tk/MainWindow.pm line 53. Tk::MainWindow->new() at /usr/local/bin/tlmgr line 4704. 

No such error message when starting it without sudo (can't use it this way though since I installed it system-wide).

Same result if

• doing sudo -H
• doing it inside sudo -i (should be the same as sudo -H AFAIK)
• using pkexec instead of sudo (apparently the new replacement for gksu/gksudo?)

Then I tried being clever and played around with the X authority file (got the idea from posts such as https://unix.stackexchange.com/a/8250):

ln -s /run/user/1000/gdm/Xauthority /root/.Xauthority # inside sudo -i then export XAUTHORITY=/root/.Xauthority 

After that I'm getting the error message with the following first two lines (and the rest the same as previously):

Invalid MIT-MAGIC-COOKIE-1 keyperl/Tk unusable, cannot create main windows. That could be a consequence of not having X Windows installed or started! 

Output for xauth list:

linux-q1xc/unix: MIT-MAGIC-COOKIE-1 fa445a3c038b19300e8e45d4ae0ae55a #ffff#6c696e75782d71317863#: MIT-MAGIC-COOKIE-1 fa445a3c038b19300e8e45d4ae0ae55a 

For sudo xauth list:

linux-q1xc/unix:10 MIT-MAGIC-COOKIE-1 7e608058d97a6bf4ae346879f4f5fd46 

For ssh -X localhost xauth list:

linux-q1xc/unix:10 MIT-MAGIC-COOKIE-1 faaf2ba1724d5efe85aa5ef6cb4fcc52 

For ssh -X root@localhost xauth list:

linux-q1xc/unix:10 MIT-MAGIC-COOKIE-1 e8168ff05f56458663f187ec38f744fe 

After sudo ln -sf ~/.Xauthority /root

Output for xauth list:

linux-q1xc/unix: MIT-MAGIC-COOKIE-1 fa445a3c038b19300e8e45d4ae0ae55a #ffff#6c696e75782d71317863#: MIT-MAGIC-COOKIE-1 fa445a3c038b19300e8e45d4ae0ae55a 

Output for sudo xauth list:

linux-q1xc/unix:10 MIT-MAGIC-COOKIE-1 faaf2ba1724d5efe85aa5ef6cb4fcc52 

Error message is again the first one.

xauth list output inside su - or using sudo -E (both where it works):

linux-q1xc/unix: MIT-MAGIC-COOKIE-1 fa445a3c038b19300e8e45d4ae0ae55a #ffff#6c696e75782d71317863#: MIT-MAGIC-COOKIE-1 fa445a3c038b19300e8e45d4ae0ae55a 

(Same as the first one.)

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Works with ssh -X localhost tlmgr -gui. Also works inside su - and using sudo -E.

What is the underlying issue and how can it be solved?