@Aftab_786,
Oh man ... that's a whole lot of unknowns when you have so many variables in play. I would 100% not move forward with anything outside of a maintenance window if things are currently working if you haven't had config sync in a year. It doesn't seem like you have traffic impacting config drift with a years worth of changes missing, but in most environments that would mean you're missing rules you would likely need. However 700 days without an update perhaps that isn't the case in your environment.
2. Manually make .136 active by lowering its HA priority from 100 to 50.
Do you have preempt enabled so that this would actually work?
4. Once .136 becomes active, perform config sync from .136 to .135.
Do you want the config from 136? If 135 is your primary and supposedly where any configuration changes would have been made, that should be your most current configuration should it not? I would be looking at the diff between peers and seeing which config you actually want to take over or at least have the knowledge needed to 'rebuild' any required changes.
Alternatively, would simply rebooting the active firewall (.135), which has been up for about 700 days, potentially resolve the login issue if the problem is related to resource exhaustion or a system fault?
You have recent experience that your failover is functional, so you do have the option of simply rebooting .135 and allowing your passive firewall to take over things and seeing if that addresses your access issues. This would be my recommendation (within a maintenance window) and then you can see if that fixes your login issues and then deal with the configuration sync problems.
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