Missing plugin "base class" with recent upgrade to 2.27.0

[acoder2020]

After upgrading to 2.27.0 from 2.26.1, I copied my plugins from the 2.26.1 branch to 2.27, all "aftermarket" plugins are there and working except this one error:

plugins

Missing Plugin Base Class

A directory for this Plugin was found, but there was no corresponding Plugin code inside of it. Make sure the directory's name matches the Plugin's base name (case sensitive)

Manual intervention required

No additional details were provided about the directory this plugin lives in. I didn't find any empty directories in my 2.27.0/plugins/ folder.

[cas]

Well the error message is pretty clear.
Every directorymust contain a php script withe name of that directory (mind the capitals).
So in case the plugin directory is called "MyPlugin" then inside that directory you need to have a file called "MyPlugin.php" .
Error states that in at least once this is not the case

[dregad]

Actually the check is a bit more complex than what cas said.

This error message is improved in upcoming 2.27.1, here is the new message:

A directory for this Plugin was found, but there is no corresponding Plugin code inside of it. Make sure that: the directory's name matches the Plugin's base name "Example" (case sensitive), a PHP file with the same name exists in the directory, and it defines a Class named "ExamplePlugin" extending MantisPlugin.