Welcome. Please avoid using all Caps when you post, especially in the title. It makes it look like you are shouting.
Secondly, this post actually belongs in the Mac OS X forum. Not sure if it will be moved there, but just for future reference.
Now, need to know some things:
1. What exact Mac model do you have?
2. What exact Mac OS are you using?
3. Have you ever done any disk cleanup/maintenance/repairs, from a software perspective? You actually can do a good amount of disk cleanup on your own, and there are some excellent programs out there (both free and not free) that can help you.
4. Are you making backups to an external device? That is
critical.
5. What third party products do you have? When you "move" to a new OS, there is the possibility that some/all of that software will need to be upgraded also.
Not sure if any of this will help, but when I did a google search of "PASSWORD_ALERT_TITLE_WITH_USERNAME_OSX Error", got some "hits", and this one does talk about it in some "detail":
https://discussions.apple.com/thread/7465756?start=0&tstart=0
Have you done any "recent" software updates/installations? Also, how long has this been going on?
