I'm sure that the problem was created by something in the update to FreePBX/Asterisk and isn't a error in FOP2. I do appreciate you taking time to help me debug it. There is a new firewall component in FreePBX, but disabling it doesn't seem to help with the error I am getting.

Running:
netstat -lnp | grep 5038
Returns:
tcp 0 0 0.0.0.0:5038 0.0.0.0:* LISTEN 3461/asterisk

Running:
iptables -vnL
Returns over 200 lines. Is there something specific in that that I should pull out or do you want it all?

After changing manager_host from localhost to 127.0.0.1 I am still getting the same error (see below).
I am logged in as root when I run the test.

Flash Operator Panel 2 - Valid License (1)
EV: error in callback (ignoring): Unknown warnings category 'FOP2::AMI2' at /loader/0x1519ad0/FOP2/AMI2.pm line 311
EV: error in callback (ignoring): Unknown warnings category 'FOP2::AMI2' at /loader/0x1519ad0/FOP2/AMI2.pm line 311
Failed to connect to asterisk - 127.0.0.1:5038
Error Message: Timed out waiting for challenge

Running:
ping localhost
Returns:
PING localhost (127.0.0.1) 56(84) bytes of data.
64 bytes from localhost (127.0.0.1): icmp_seq=1 ttl=64 time=0.048 ms

Running:
cat /etc/sysconfig/fop2
Returns:
OPTIONS="-d -c /etc/asterisk/fop2"

Running:
cat /etc/asterisk/manager.conf
Shows "enabled = yes" (see full contents below)

Rebooted server, but --test returns the same errors:
Flash Operator Panel 2 - Valid License (1)
EV: error in callback (ignoring): Unknown warnings category 'FOP2::AMI2' at /loader/0x1692ad0/FOP2/AMI2.pm line 311
EV: error in callback (ignoring): Unknown warnings category 'FOP2::AMI2' at /loader/0x1692ad0/FOP2/AMI2.pm line 311
Failed to connect to asterisk - localhost:5038
Error Message: Timed out waiting for challenge

Could the deny/permit settings in manager.conf be a problem?

cat /etc/asterisk/manager.conf
;
; AMI - Asterisk Manager interface
;
; FreePBX needs this to be enabled. Note that if you enable it on a different IP, you need
; to assure that this can't be reached from un-authorized hosts with the ACL settings (permit/deny).
; Also, remember to configure non-default port or IP-addresses in amportal.conf.
;
; The AMI connection is used both by the portal and the operator's panel in FreePBX.
;
; FreePBX assumes an AMI connection to localhost:5038 by default.
;
[general]
enabled = yes
port = 5038
bindaddr = 0.0.0.0
displayconnects=no ;only effects 1.6+

[admin]
secret = amp111
deny=0.0.0.0/0.0.0.0
permit=127.0.0.1/255.255.255.0
read = system,call,log,verbose,command,agent,user,config,command,dtmf,reporting,cdr,dialplan,originate,message
write = system,call,log,verbose,command,agent,user,config,command,dtmf,reporting,cdr,dialplan,originate,message
writetimeout = 5000

#include manager_additional.conf
#include manager_custom.conf

I ran:
chmod a+x /etc/asterisk/fop2/*.sh

Now testing with:
/usr/local/fop2/fop2_server -c /etc/asterisk/fop2 --test
returns:
Flash Operator Panel 2 - Valid License (1)
EV: error in callback (ignoring): Unknown warnings category 'FOP2::AMI2' at /loader/0x1baa9a0/FOP2/AMI2.pm line 311
EV: error in callback (ignoring): Unknown warnings category 'FOP2::AMI2' at /loader/0x1baa9a0/FOP2/AMI2.pm line 311
Failed to connect to asterisk - localhost:5038
Error Message: Timed out waiting for challenge

Users still cannot log in.

Checking version with:
/usr/local/fop2/fop2_server -v
returns:
fop2_server version 2.27

I have no particular reason for using /etc/asterisk/fop2. I assume that is where the original installation put the files or instructed me to put the files.

After updating FreePBX to 10.13.66 and Asterisk to 13.5, users cannot log in to FOP2. When testing with
/usr/local/fop2/fop2_server -c /etc/asterisk/fop2 --test
I get the following output:
Flash Operator Panel 2 - Valid License (1)
Can't exec "/etc/asterisk/fop2/amipasswd.sh": Permission denied at script/fop2_server.pl line 579, <$CONFIG> line 271.
Can't exec "/etc/asterisk/fop2/autoconfig-users-freepbx.sh": Permission denied at script/fop2_server.pl line 579, <$CONFIG> line 271.
EV: error in callback (ignoring): Unknown warnings category 'FOP2::AMI2' at /loader/0x24909a0/FOP2/AMI2.pm line 311
Failed to connect to asterisk - localhost:5038
Error Message: Timed out waiting for challenge

Does anyone have advice on how to correct this?

I have uninstalled the FOP2 Admin module and installed FOP2 Manager. The FOP2 page appears to be working normally, but when I go to the fop2/admin page, there is a message saying:
"FOP2 Server is not responding. Be sure the service is running!"

I recently upgraded FreePBX to v12.0.13. Everything is working, but it shows this security warning:
Module "FOP2 Admin" is unsigned and should be re-downloaded

How can I get FOP2 Admin signed?