Up until today I have been having the same issue, hanging on splash screen.
I tried all sorts of things including many of which were listed by skylum, up to but not including 'add exception to windows firewall', since turning off my anti-virus didn't work and I was tired of trying, so I gave up for awhile.
Today however I tried a simple test.
Open windows 'Firewall and network protection' and turn off all 3 levels of protection - Domain Network, Private Network, and Public Network.
Once this was done I was able to launch Luminar 4.3 multiple times with no issues.
Then I started by activating (starting from the bottom up) each of the firewall options one at a time and testing the launch. Public Network (test a few times), Private Network (test a few times), Domain Network (test a few times).
Now with all 3 of these turned back on, and my anti-virus active (never turned it off in this process), I am fully protected and Luminar 4.3 was working fine.
I testing launch functionality from Lightroom and had no issues, but when I tried to launch the filter from Photoshop 2020, no luck.
To fix photoshop, I started Luminar as admin, then did the photoshop plugin un-install, then imediatly afterward (no reboot) I clicked the install. Then fire up photoshop and test the filter and it works fine.
Worried that the plugin re-install may have been a factor in Luminar not launching I test launched it a few more times and no problem.
Not sure why this worked (perhaps it triggered windows to add the exceptions to the firewall rather than me doing it manually?). But it did work, so I am posting this in the hopes that it may help someone else.
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