Why does my Mac mini sometimes not accept the correct password?

Does this explain your issue: Sometimes when I boot and fill the login password, the bluetooth keyboard is not fully wake and wakes up only after typing the 1st password letter. So the entered password missing the 1st letter is wrong. So I have a habit verifying that the 1st password letter is really entered, if not, I type it again and then the rest of the password.

By chance do you have ANY of the following types of third party apps installed on the computer?

• Antivirus
• Security
• VPN
• Cleaning
• Maintenance

It will help us get a good idea of what is causing your problem if you provide a report of your system, that way we do not have to play 20 questions with you. The report we are requesting does NOT provide any personal information and is extremely safe to use. Please navigate towww.Etrecheck.com and download the free version of EtreCheck. Once you have you downloaded the app and installed it, please run the report and save it. This report will help us get a good idea what has been installed on your system and help us be able to diagnose what may be wrong.

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I concur with Matti and also think that the keyboard, needs more time to fully wake and connect.

Are you re-waking the Mac mini with the keyboard or mouse...?

Test lock screen and display sleep, instead of full computer sleep.