thereitis thank you for the report. This is very unusual as the line of code that is causing this error should ever produce anything like.
It almost like the user option
aam_user_expiration contains not a string but serialized object DateTime, which AAM does not store any serialized objects in the database.
You can reset all AAM settings or delete only
aam_user_expiration records in the wp_usermeta database table. This should do the trick.