Hi,
Installing fop2admin or not, or any other freepbx module, cannot produce an error as "unable to connect to asterisk manager". It seems you have a different manager password set in /etc/asterisk/manager.conf than the one that you have in /etc/amportal.conf or similar file.
It is totally unrelated to fop2admin or fop2. It is a freepbx issue. In any case, be sure to install the latest FOP2Admin (1.2.13) that will work fine on recent FreePBX versions, and remember that the Queue and Park modules are prerequisites, you have to install them first.
Best regards,