I would verify you copied all the new information in properly. Make sure you can log into mysql as that user
mysql -u YOURPIWIKUSER -p
You could try restarting mysql, make sure you flushed the changes. To me that error means something is passing a bad password. Make sure the password changed for the correct section in the config. Verify your DB setup and the superuser section. Also I would try restarting your webserver or reloading the config. The link you posted is showing that your original error is still there.
Though again if this had nothing to do with the update what I stated earlier may not be in relation to your actual issue. maybe grep your files for your old password. But I think its either copied wrong, changes were not updated or something else with mysql as its trying to authenticate but not using the proper password. Are you sure you changed the correct password in mysql?
Wish I could be of more help.