Invalid serial number message after installing on new computer

I just bought a new computer (Windows 10) and installed X5 on it (downloaded version, not from a CD). At the beginning of the installation process, when it asked if I had a serial number, I entered the one I got at the time of purchase, which worked fine on my old computer. When I open the program, it takes me to the activation screen, but when I click on Activate Now, I get the error message that my serial number is invalid. There are two possible reasons I can think of for this, as I will explain below.

1) When entering my serial number, I copied the number from the About CorelDraw screen on the old computer's installation, but it did not have the dashes, so I thought that might be the reason for the error message, so I uninstalled X5 on the new computer, then reinstalled using the number with dashes, which I retrieved from my account page on Corel.com. Unfortunately, that didn't solve the problem. I am aware that, even when you uninstall a program, some files and registry entries can get left behind, so I thought that perhaps somewhere on my system is something that still has the serial number without the dashes. And, yes, I did put the checkmark in the box for removing profiles and other personalized settings when I uninstalled. So, if this is the problem, where could I find that pesky file or entry that has the incorrect serial number so I can delete it and try another reinstall?

2) My version of X5 was purchased as an upgrade, my previous version being X3. On my old computer, X3 is still installed, but it isn't installed on this new one. I seem to recall that, in the past, when I bought an upgrade version and didn't have the old version on my system, it would ask for the serial number of the old version to confirm that I qualified. No such question was asked when I installed X5 on my new computer. I have my doubts that this is the problem because I also installed Corel WordPerfect X7 on the new computer, and it works fine even though it is also an upgrade version, and the old version isn't on the computer.

I did not get a message that I have reached the maximum number of activations, so I don't think that's the problem. Between the old computer and the new one, I have two copies presently being used. Eventually, my old computer will be put away somewhere and used only when I need to use a program installed on it that won't work in Windows 10.

If anyone can help me, it would be greatly appreciated. Though I can use the program without activating it for now, that will only work for 30 days, so I really need to get this resolved. Since I know that someone will probably ask, no, I am not interested in upgrading to X7. I took a look at it with a trail download, and though CorelDraw X7 is fine, Photo-Paint X7 is not, for reasons I won't go into here since it's off-topic. One of these days, I'll go looking to see if there is a Corel feedback and complaint form or forum so that I can tell them what my issue is with Photo-Paint X7.