Hey there,
not sure if this might have nothing to do with Stackideas products, but I have never gotten a similar problem before using software from here.
I made the update by writing files directly and it stuck at 86,3% and I just waited until there was a error 500. Later on I found out that I needed to repair the database. Did that and now everything is working, probably.
Do you have any suggestion about how to make it better? Because the above way I used on my test site, my production site is still on 3.4.8.