Superb.You helped me out big time. It was the modsecurity.
For others who have the same problem. This is how you can fix it:
Go to DirectAdmin:
-> Advanced Features
-> ModSecurity
-> Turn setting “Off`” and save.
I have the exactly the same question from 3.8.5 to 4.5.2. I tried to use the recommended upload plugin and manual ftp (including deleting the old theme). But I get a message: ERR_CONNECTION_REFUSED
I also tried it with an Enfold version from feb 2018. Same happened.
UPDATE:
It now works but slowly.
thank you!