Had the same problem (stalling at 25% or 30%).
This worked for me:
1. Reboot computer or use Activity Monitor to force quit any prior Adobe Flash installation process that may be still running
2. Download Adobe Flash update using this link (assuming you are using Safari or Firefox): Flash Player for Safari and Firefox - NPAPI
3. Mount flash player by clicking on flash drive icon on desktop
4. Right click on Flash file
5. Choose 'Show Package Contents'
6. Open 'Contents' then 'Resources'
7. Run file 'Adobe Flash Player.pkg'
8. Reboot computer.
This fixed it for me. Thank you Lexillix!
For me it would start downloading and then the download progress bar would disappear. I was stuck with a grey box. Your work around worked great!
This morning we resolved a problem that prevented Flash Player installations from completing for some of our Macintosh users.
If you encountered this problem, please delete any previously downloaded Flash Player installer and either:
1.) Download the installer again from https://get.adobe.com/flashplayer
2.) Download the stand-alone installer from http://fpdownload.macromedia.com/pub/flashplayer/latest/help/install_flash_player_osx.dmg