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This is nearly always caused by either a javascript conflict but - more commonly - because another process is hijacking the AJAX call. Some login authentication plugins can do this - for example - because they monitor the ajax requests to the server and then hook extra code in. This means that the response we get back from the server is not what is expected and it terminates.

So - if you have performed successful SP upgrades before then you need to work out what had changed to your website. Any new plugins introduced - for example.

Alternatively if you know of any plugin you have installed that might be sending out extra data with ajax requests then deactivate it while you perform the upgrade.

The other possible way is to (1) Try it with the WP default theme and if still no joy (2) deactivate other WP plugins one at a time until it goes through. At least that way we will know where the issue stems from. Actually - if you place your site in Maintenance Mode it really doesn't usually take very long to do this.

I started by saying 'nearly always'. I would say probably 95% of  the time it is one of the above.