Release the button immediately when the phone's LED indicator turns and the pop-up window disappears.
Version 0.9.185.0 represents the last major stable release before significant architectural changes were attempted in later versions (which often introduced bugs). flashtool 09185 best
"I maintain a collection of FTF files for the Xperia Z2. I have tried every version after 09185 to bundle sin files, and they always fail checksum. I keep an old Windows 7 VM just to run 0.9.18.5 for bundling. It's the best." — Release the button immediately when the phone's LED