What are the causes of permission errors for /run/user/NNNN/dconf/user files? How can we resolve them?

There are several open questions regarding GUI apps whose console output shows repeated errors similar to the following:

(XXXX:YYYY): dconf-CRITICAL **: unable to create file '/run/user/NNNN/dconf/user': Permission denied.  dconf will not work properly.

or

(XXXX:YYYY): dconf-CRITICAL **: unable to create directory '/run/user/NNNN/dconf': Permission denied.  dconf will not work properly.

Where XXX is an application name, YYY is a number, and NNN is a user id number (?). * https://unix.stackexchange.com/q/665025/34868 * https://unix.stackexchange.com/q/399323/34868 (due disclosure: An issue of mine from 3 years ago) * https://unix.stackexchange.com/q/82974/34868 * https://unix.stackexchange.com/q/278797/34868 * https://unix.stackexchange.com/q/652444/34868 And these often remain unanswered, or with answers which don't resolve the issue (or possibly unresponsive posters). I was hoping to attract enough community attention to this question (perhaps with a bounty if necessary) to provide proper description of the possible causes of this issue, and a list of several actions which should resolve, say in 90% of cases, so that in the future one could easily mark yet another variant of these questions as a duplicate of this one.