Therefore I checked this board here and found out that the esellerate.dll has to be in the Windows folder (WinXP) and in the FS main folder. Last one was not done, either by the installer of O'Hare and the extra download of the addon manager. Therefore I had to copy the esellerate.dll manually to the FS9 main folder to make the registration of the serial work.
Sorry, but this is not what has been explained on the forum.
In fact, is exactly the opposite: the Esellerateengine.dll should be put ONLY in the C:\Windows folder (only under XP), and not in the FS9 folder, which is what the installer does. Not only that: it also REMOVES any Esellerateengine.dll that is found outside the correct folder for the OS version.
This, doesn't mean that, if you manually put the dll in the FS9 main folder, it wouldn't work, but it's not the best setting, becasue if that dll gets updated (and correctly put into the C:\Windows folders), the one residing in the FS9 root folder will take precedence over it, barring the chance to be updated. So, as long as the one in the FS9 IS the last one, it would work, but in any case Esellerate updates it (the .DLL is also able to self-update), the update wouldn't work.
So, it might have worked for you because you had an OLD version of the Esellerateengine.dll in your C:\Windows folder and, by putting the NEW one on the FS9 folder, you forced FS9 to use that one as well, but it would have been exactly the same if you overwrote the one in C:\Windows.
So, the real issue is why our installer wasn't able to update the C:\Windows copy instead, because is supposed to do that. This might have happened because of either the file being already opened and locked (sometimes FS9 doesn't close properly and release all the locks on files) and that should have probably been fixed by a reboot before installing.
OR, you have a permission issue, so the installer, for some permission related issue, can't write into your C:\Windows folder. This might have been probably fixed by running the installer as admin, although the installer IS already flagged to request admin permission, so this is normally done automatically, but on some specific permission policies, it might not always work.
NOTE: there might be another software out there using Esellerate, that is installing the DLL into C:\Windows\Sytem32, which is wrong as well. We had another user that wasn't able to activate, and eventually found another copy of the Esellerateengine.dll into C:\Windows\System32, and fixed the problem by removing it.
So, ideally, the correct solution would be searching the whole harddrive for all copies of this DLL, and remove them ALL, then run any of our installers again, to let it put the last one into C:\Windows, which is the one and only correct place, ON XP. On Vista it's C:\ProgramData\Esellerate.