[SOLVED] user '_apt' permission issue during installations

[Sadi]

I get the following error message after software installations via apt/synaptic:

W: Download is performed unsandboxed as root as file '/var/cache/apt/archives/partial/.........deb' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)

I wonder if this might be due to the respective user entry written in the file "/etc/passwd" during system installation as "_apt:x:105:65534::/nonexistent:/bin/false" and if the solution could be replacing "/bin/false" with "/usr/sbin/nologin"?
EDIT: No, this doesn't fix it.

[Sadi]

Apparently, this access permission issue occurs when, for example, user decides to store package files downloaded by apt in another partition, creating a directory there as root, and symlinking it to "/var/cache/apt/archives".
The problem persists even if the user '_apt' is specifically given ownership of subdir "partial".
Therefore, this must be either so by design or an upstream bug, I guess.