Getting this error:
"You have specified an incorrect password. Please check your password and try again. If you continue to have problems please contact the Board Administrator"

Solution:
1. try manually deleting everything from your /cache folder EXCEPT index.htm and .htaccess using FTP.
2. Try to use other browser after deleting cache files form phpBB3 web server. Eg. from Edge to Chrome.
3. make sure the password is correct. Else, try to use phpmyadmin to reset it.