You've cleared browser cache and cookies?
You've cleared server cache by going directly to url:
https://yoursite/admin/purgecaches.php
Are you sure you have looked in the lib/editor/tinymce/plugins/ for a folder/directory called tinymce_iassign. If it's not there, refreshing that screen should show missing from disk. Go ahead and click whatever button at the bottom of that screen to update the database tables.
What happens if you click install?
Is there any button to update the database lower on that screen?
Am pretty sure I asked this once ... but will again ... do you have a backup of code directory and DB that dates prior to the errant attempt to install this plugin?
'SoS', Ken