This has been a recurring issue/bug with Samsung Phones, ie, for some strange reasons even w/o changing the security settings, the phone just lock out and shows the message:

"Your password contains at least 4 characters, including at least 1 letter"

Read similar experiences from other members in the community and it appears samsung has not addressed this at all to the complete satisfaction (ie, not factory reset & completely erase data) of the customer for YEARS already. 

Appreciate any working lead from anyone - please be kind not to blame or assume we don't own our phones. Don't be cruel, we're here to ask for help not to read your nonsensical comments. Thank you.