Sorry for the delayed response, only today I had a little time to update my High Sierra installation to Security Update 2020-003. I had iTunes 12.6.4.3 installed, so I downloaded iTunes 12.6.5.3 from
https://support.apple.com/HT208079 and tried to install it. First I got this message about the certificate being expired, but the installation continued and was successful.
View attachment 921710
Starting iTunes, I got “An unknown error occurred (-42408)” then, when I tried to log in, I got “An unknown error occurred (-50)” and again (-42408). I was unable to log in after multiple attempts. On another account on the same installation that was logged in, iTunes displayed the same -42408 error message.
View attachment 921711
View attachment 921712
I’ve tried to use the script from post 384 (
https://forums.macrumors.com/thread...tible-with-macos-mojave.2143244/post-28399645) to change iTunes 12 to 13, but it did’t make any difference.
Did you have better luck?
Let’s be honest, all I did is some fancy copy-paste and text edit

I have an idea about solving the current problem, but it’s beyond my abilities. I’ve asked for help, but so far no reply.
https://forums.macrumors.com/thread...e-iphoto-or-itunes-on-macos-catalina.2207898/
In this thread
https://forums.macrumors.com/threads/itunes-12-6-5-3-iphone-11-ios-13-4-not-syncing.2229520/ GBstoic posted a solution for the sync issues between iTunes 12.6.5.3 on Windows and iOS 13 devices.