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We really are going to need a lot more detail please.

What actions have you taken (step by step)? What messages are you seeing on the screen? Anything else that may help...

This can happen if the http request (which is using an AJAX call) gets hijacked by another process. This would usually mean a plugin is attempting to ride on the back of the request causing the return message to fail.

Another possible cause - although more rare - is of your hosting company has put some kind of security on your system that does not recognise the call but this is more unlikely.

There are 2 possible ways around it. IF each time you try and run the upgrade the message 'current build' changes the number (i.e., it gets bigger) - then you could keep running the upgrade until it finishes. This might work. However, as you are upgrading from several older versions I calculate you would probably need to do it about 22 times!

Or - of course - you could track down the conflict. Running the upgrade with the browser web console open to show script errors might tell us the conflicting component but it may not. Deactivating any plugin that is new since you last upgraded SP while you perform the upgrade is another way of identifying the problem and getting the upgrade finished. Every now and then - but again rare - it is the Wp theme that is the problem. But as I say - this is very rare.

I appreciate my reasoning and explanation on what is likely wrong might be a bit 'geeky' or techie... but the description of how to work around it wasn't - I do not think. It can usually be overcome easily and quite quickly when it happens.