DMG and PKG File Verification Stopping at 60% on Multiple Files After Upgrading 2019 27" 5K iMac to macOS Sequoia 15.4.1

Hello, I have encountered a very curious problem which I don't know how to resolve.

I have a 2019 27" 5K iMac.

Recently, I had the original fusion drive removed from the iMac -- because the HDD was fried -- and replaced with a 4TB SSD. This was done by an authorized Apple redistributor.

After macOS Sequoia 15.3.2 was installed on the new 4 TB SSD, I ran Software Update to upgrade to 15.4.1.

Next, I tried using Migration Assistant multiple times, using a thunderbolt cable, to try to migrate my user data from my 2023 Apple Studio to my 2019 iMac. However, after trying to get it to work for two whole days, I finally gave up, because the migration kept stalling.

So, it was at that point that I decided to use Carbon Copy Cloner instead to clone my Apple Studio user folder to the 2019 iMac from an external USB backup drive which had been connected to the Apple Studio.

To be clear, I disconnected the backup drive from the Apple Studio and connected it to the 2019 iMac instead, and then I used CCC to clone the backup to the iMac, which was now running macOS Sequoia 15.4.1.

While most of my apps seem to work, I have run into TWO problems.

1. First, I need to reinstall a few apps. However, whenever I double-click on either a DMG file or a PKG file so that I can install an app, the OS verification process immediately goes up to about 60% and then it just stops and doesn't proceed any further. I have tried this on different DMG and PKG files, and the result is exactly the same every time. I even tried using that quarantine command in the Terminal app, and that didn't help either.

2. The second problem is that several apps say that they can't be run on the iMac, even though they ran just fine on it before. Not only that, but these same apps continue to run just fine on the Apple Studio from which they were backed up, and then cloned to the 2019 iMac. Again, BOTH machines are running macOS Sequoia 15.4.1.

So I am not sure what is going on with either of these two problems. I am guessing that it has something to do with either permissions, or maybe some new security thing Apple has done in 15.4.1, but that is just a guess.

Any help would be greatly appreciated. Thanks in advance.