$config['enableListeners'] = false;
I had faced a similar issue...
Once, you would disable the listeners, it would disable all add-ons interfering with the core xF setup.
To disable add-ons, Edit your config.php file on your server and add the following code at the end;
$config['enableListeners'] = false;
Now, you should be able to login to your admincp;
Now, copy 2.5.9 files on to the server, overwriting the 2.6.3 files;
Now, uninstall this add-on from your admincp;
Now, delete files associated with this add-on from your server;
Now, ask you webhost to upgrade to php 7.2, make sure no other add-on clashes with php 7.2;
Now, copy 2.6.3 files on to the server;
Edit config.php file as follows:
$config['enableListeners'] = true;
Now, install the add-on using the newest xml file as usual;
And live happily ever after!
