So to answer my own question based on the comments:

The problem was that the 'old' server has a locale LANG=en_US.UTF-8 bu the new server has locale LANG=nl_NL.UTF-8 (on Linux level). So I added the locale:

<format property="1_day_ago" pattern="MM/dd/yyyy hh:mm a" locale="en,US" offset="-1" unit="day" />

and now it is working as expected.

Note also the correct pattern is 'MM/dd/yyyy hh:mm a' and not 'MM/dd/yyyy hh:mm aa'.