Using version 2021.001.20155 on Windows 10, I have encountered several bugs:
Things I've tried:
Hope you are doing well and sorry for the trouble. As described, Scroll bars are never shows and Reader always launches with the toolbar hidden
As described you have already tried the troubleshooting steps shared above with no success. Please try to reboot the computer once and see if that helps.
If it still doesn't work, please reset the Acrobat Preferences to default as described in the help page - https://community.adobe.com/t5/acrobat/how-to-reset-acrobat-preference-settings-to-default/td-p/4792... and see if that makes any difference.
if it still doesn't work, please create a new test user profile with full admin rights in Windows or enable the root account in Mac and try using the application there and check.
This appears to be an issue with a specific document. Other documents do not exhibit the same issues.