Solaar notes this on configuration:

However, even if "yes" is selected, the line that includes plugdev is
commented out in the udev rules file shipped with the package,
/lib/udev/rules.d/60-solaar.rules:

# Grant members of the "plugdev" group access to receiver (useful for SSH users)
#MODE="0660", GROUP="plugdev"

...which results in plugdev not being set as the group for matching
devices, instead leaving them with the default 0600 root:root, which
plugdev users cannot access. If this line is manually uncommented Solaar
works as intended with plugdev users.

The easiest fix might be to simply be to note this in the configuration
dialog, as it'd be reasonable for users wanting this to work with plugdev
to understand that an additional manual step is required.

Observed on Debian Bullseye.

We believe that the bug you reported is fixed in the latest version of
solaar, which is due to be installed in the Debian FTP archive.

A summary of the changes between this version and the previous one is
attached.

Thank you for reporting the bug, which will now be closed.  If you
have further comments please address them to 1028922@bugs.debian.org,
and the maintainer will reopen the bug report if appropriate.

Debian distribution maintenance software
pp.
Stephen Kitt <skitt@debian.org> (supplier of updated solaar package)

(This message was generated automatically at their request; if you
believe that there is a problem with it please contact the archive
administrators by mailing ftpmaster@ftp-master.debian.org)
Format: 1.8
Date: Tue, 17 Jan 2023 13:25:03 +0100
Source: solaar
Architecture: source
Version: 1.1.8+dfsg-2
Distribution: unstable
Urgency: medium
Maintainer: Stephen Kitt <skitt@debian.org>
Changed-By: Stephen Kitt <skitt@debian.org>
Closes: 1028922
Changes:
 solaar (1.1.8+dfsg-2) unstable; urgency=medium
 .
   * Restore the plugdev setup which was accidentally removed in
     1.0.1+dfsg-4. Closes: #1028922.
   * Standards-Version 4.6.2, no change required.
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