Starting with this issue, you want to check the contents of
/etc/asterisk/manager.conf
and ensure that the admin user and password for FreePBX are set correctly. You can do this in Advanced Settings if you have GUI, or otherwise via the CLI with:
# cat /etc/asterisk/manager.conf
*snip*
[admin]
secret = new_password
*snip*
# fwconsole setting AMPMGRPASS new_password
Changing "AMPMGRPASS" from [old_password] to [new_password]
# fwconsole setting AMPMGRUSER admin
Changing "AMPMGRUSER" from [admin] to [admin]
Once things are working agan, switch to Asterisk 16