I am getting these errors at a clip of 824 Warnings per second!

Failed to open stream: No such file or directory

line 201 in class-piklist.php is

include_once self::$add_ons[$addon]['path'] . '/includes/' . $include;

What I don’t understand are these non-existing files, such as wp-login.php.2018-06-14_103024.bak something is trying to get access to.

Thanks Steve for getting back to me.
I added a screenshot of what I have in the Add-ons page. It looks like nothing is added on.
I did write a Piklist plugin a while back which is activated. Could this be causing the problem?
I also have the Classic Editor plugin active.

I turned my created Piklist Plugin off, and these errors kept coming in. It is only after turning Piklist off all together that the errors stopped.

I do not.

Thank you Steve.

I removed all my plugins and updated to WP 5.8.2. I also made a windows system file check — windows found some error and auto-fixed them.

Having piklist removed for some days made these error go away. To note is that I had two sites using piklist. Separate installs on separate databases. When I uninstalling one, the other site started recording errors. So I uninstalled everything that is piklist related.

I now reactivated piklist and installed my piklist metabox plugin (just for one site.) It works, and I get no errors for now.

I do not know what could have caused these errors. I also could never reproduce them myself. They just started showing up in the windows error logs at a rapid clip. I will let you know if they return.

At the moment I am getting a lot of Active Server Pages: Unexpected error. A trappable error (C0000005) occurred in an external object. The script cannot continue running.. errors.

Just before IIS Application pool went down for a good long time — all (C0000005) errors; I got the following error, but I am not sure if it is at all related to the (C0000005) errors or the piklist errors I had:

The description for Event ID 2 from source PHP-8.1.0 cannot be found. Either the component that raises this event is not installed on your local computer or the installation is corrupted. You can install or repair the component on the local computer.

If the event originated on another computer, the display information had to be saved with the event.

The following information was included with the event:

php[14144]
PHP Deprecated: trim(): Passing null to parameter #1 ($string) of type string is deprecated in D:\piwik\core\CliMulti\Process.php on line 290

The locale specific resource for the desired message is not present