Cisco ASA 5506x ASDM issue - stuck on "Software update completed" but only one 1 out of 2 devices.

I have a active /standby pair of ASA 5506 running the following software;

firewall1(config)# sh ver

Cisco Adaptive Security Appliance Software Version 9.8(4)20
Firepower Extensible Operating System Version 2.2(2.124)
Device Manager Version 7.13(1)

Compiled on Thu 02-Apr-20 10:19 PDT by builders
System image file is "disk0:/asa984-20-lfbff-k8.SPA"
Config file at boot was "startup-config"

firewall1 up 6 days 0 hours
failover cluster up 5 years 249 days

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firewall2# show ver

Cisco Adaptive Security Appliance Software Version 9.8(4)20
Firepower Extensible Operating System Version 2.2(2.124)
Device Manager Version 7.13(1)

Compiled on Thu 02-Apr-20 10:19 PDT by builders
System image file is "disk0:/asa984-20-lfbff-k8.SPA"
Config file at boot was "startup-config"

firewall2 up 1 year 120 days

failover cluster up 5 years 249 days
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When firewall1 is active, I am unable to connect to ASDM, but I can connect to ASDM on secondary IP.
When firewall2 is active, I am able to connect to ASDM, but I am unable to connect to ASDM on secondary IP.

The issue is that it always gets stuck on "Software update completed." and will go no further. I have read a number of issues with versions of JRE etc, but I am at a loss of why it would work with only 1 of my firewalls when they are both identical. I have tried removing the local cache folder and even reinstalling ASDM launcher, but same issue persists.

I will be in a position to update the software if this might stop the problem, but again not sure why only 1 machine.

Thanks