BIGIP 12.1 GUI unable to alter node/pool settings, error 1137

Answer

From the link https://support.f5.com/csp/article/K16873079:

Known Issue The BIG-IP system may fail to bring a pool member offline.

This issue occurs when the following condition is met:

You attempt to set the state of a pool member to user-down using the following Traffic Management Shell (tmsh) command syntax: tmsh modify /ltm node state user-down

For example, to set the state of pool member 192.168.10.10 to user-down you would type the following command:

tmsh modify /ltm node 192.168.10.10 state user-down

Workaround:

1) SSH to your F5 appliance:

2) Go to bash using command:

user@(BIGIP)(cfg-sync In Sync)(Active)(/Common)(tmos) bash
[user@BIGIP:Active:In Sync] ~ 

3) Backup your bigip.conf file:

[user@BIGIP:Active:In Sync] ~  cp bigip.conf bigip.conf.$(date +%Y%m%d%H%M%S).bak
[user@BIGIP:Active:In Sync] config  ls -al bigip.conf*
-rw-r-----. 1 root root     593408 2019-01-23 09:50 bigip.conf
<--- output omitted for brevity --->
-rw-r-----. 1 root webusers 593408 2019-01-23 10:14 bigip.conf.20190123101439.bak

4) vi your bigip.conf file

[user@BIGIP:Active:In Sync] config  vi bigip.conf

5) Navigate to the affected VIP location in bigip.conf, in this instance it's YOUR_POOL and the affected pool member is server3.local

ltm pool /Common/YOUR_POOL {
    members {
        /Common/server1.local:443 {
            address 10.1.2.3
            session user-disabled
        }
        /Common/server2.local:443 {
            address 10.2.3.4
        }
        /Common/server3.local:443 {
            address 10.3.4.5
            monitor /Common/your_wrong_monitor
        }
    }
    monitor /Common/your_smart_monitor
}

6a) IF your problem is related to a pool monitor erroneously applied Delete the line:

session user-disabled

6b) OR if your problem is due to the KB listed above then delete the line Delete the line:

monitor /Common/your_wrong_monitor

7) Save and close the file, now verify the bigip.conf using the following command

[user@BIGIP:Active:In Sync] config  tmsh load /sys config verify

😎 If your bigip.conf file successfully verifies, load the file into your running config using the following command:

[user@BIGIP:Active:In Sync] config  tmsh load /sys config

Now you should be able to alter your nodes in the affected pool. Be sure to sync your configs to other devices in your group.