“/function.is-dir” – Plugins Absent to WP Install

Hi, new to this forum and to Word Press. I had it installed a few days ago to replace a functioning Movable Type install and everything has been functioning well.

UNTIL I installed a number of plugins.

Up until yesterday, each of the Plugins functioned or failed and I was able to successfully remove the ones that failed to function (incompatible with current WP install, etc.).

HOWEVER, today I accessed my WP Admin. panel and when I selected the “PLUGINS” tab in the Admin. panel, I got the following wall of errors response:

…/wp-admin/function.is-dir

(Page Not Found)
and this at the bottom of the Admin. panel”
“You do not appear to have any plugins available at this time.”

I checked my cpanel and see all the previously functioning, installed Plugins still in place. They worked yesterday, nothing’s changed to my knowledge since then, but today, this error and declaration that I have no Plugins installed (not so).

WP is not reading the existing Plugins, obviously.

However, yesterday I experienced a number of announcements when accessing Plugins advising me that a few couldn’t “read” or write to files due to Permissions problems.

I am having a great deal of problems understanding how to set Permissions in my cpanel array of directories and files AND I have great concerns about security vulnerabilities when trying to accommodate WordPress demands/requirements for full write-to permissions on certain directories/files.

THUS, could someone please help with these two related problems?

The first would be:
(1.) https://mydomain.com/DIRECTOR-W/BLOG/wp-admin/function.is-dir
= (Page Not Found)
“You do not appear to have any plugins available at this time.”

The second would be:
(2.) Setting Permissions so WP Plugins can write to/properly function with my existing Directories and the WP install with additional plugins properly installed.