Error at 100%

First off, thanks for an awesome plugin – I’ve used this on quite a few sites and never had any issues until today!

The current upload is stuck at 100%, I have tried several recent versions of the plugin and also rolled back WordPress to v4.9.9 to see if it makes a difference.
It does not ??

I read through your guide about being stuck at 100%, I have checked the php.ini settings and all looks good!

The console shows the below errors:

admin-ajax.php:1
Failed to load resource: the server responded with a status of 404 (Not Found)

import.min.js?v=6.82&ver=4.9.9:1384

Uncaught Error: error
    at Object.error (import.min.js?v=6.82&ver=4.9.9:1384)
    at i (load-scripts.php?c=1&load[]=jquery-core,jquery-migrate,utils&ver=4.9.9:2)
    at Object.fireWith [as rejectWith] (load-scripts.php?c=1&load[]=jquery-core,jquery-migrate,utils&ver=4.9.9:2)
    at y (load-scripts.php?c=1&load[]=jquery-core,jquery-migrate,utils&ver=4.9.9:4)
    at XMLHttpRequest.c (load-scripts.php?c=1&load[]=jquery-core,jquery-migrate,utils&ver=4.9.9:4)

Any ideas / suggestions?
Please bear in mind I have tried the steps in your sticky post, if you feel i should examine one further please so say which!

Thanks in advance,
Alex