Thunderbird does not startup when ~/.thunderbird is symbolic link

[Manfred Steiner]

lsb_release -rd
Description: Ubuntu 21.04
Release: 21.04

apt-cache policy thunderbird
thunderbird:
  Installiert: 1:78.8.1+build1-0ubuntu1
  Installationskandidat: 1:78.8.1+build1-0ubuntu1
  Versionstabelle:
 *** 1:78.8.1+build1-0ubuntu1 500
        500 http://at.archive.ubuntu.com/ubuntu hirsute/main amd64 Packages
        100 /var/lib/dpkg/status

Bug description:
Up to now I use ecryptfs for a private encrypted directory ~/Private, move the ~/.thunderbird folder to ~/Private and create a symbolic link ~/.thunderbird -> ~/Private/.thunderbird.

Working fine in the past, but now Thunderbird popups a message dialog:
"Thunderbird is already running, but is not responding. To use Thunderbird, you must first close the existing Thunderbird process, restart your device, or use a different profile."

Error message is wrong, because there are no processes running. Starting from terminal shows:
"JavaScript error: resource://gre/modules/XULStore.jsm, line 66: Error: Can't find profile directory."

When starting with option "--safe-mode" no warning is printed.

When moving folder ~/Private/.thunderbird back to ~/.thunderbird (removing the symbolic link before), Thunderbird starts as desired without error.

Same effect when creating a new clean ~/thunderbird. As soon as it is moved to ~/Private and using a symbolic link instead, Thunderbird does not startup.