When using the Moodle Add-ons installer from the Moodle site administration menu, if the uploaded plugin zip file exceeds the site's max upload file size, the error reported is:
A required parameter (sesskey) was missing

Several users complained in the comments on the PoodLL filter (18MB ). So I did a little investigation. To reproduce the problem:
i ) set the site max upload file size to 2M (at site admin->security->site policies)
ii) Go to the add-ons installer. (at site admin->plugins->install add on )
Choose "text filter" for the plugin type.
iii) Drag a zip file of greater than 2MB into the file manager. (you might use the poodll filter zip from here: https://moodle.org/plugins/view.php?plugin=filter_poodll )

After the file has uploaded the error reported will be "A required parameter (sesskey) was missing"

I believe the error should be something along the lines of "max file upload size exceeded"