@Hias: Thank you very much for your response!
I've done what you said and disabled the LIRC translation via the command you gave, and after that, "mod=longpress" is working again! With the help of the "Keymap Editor" addon it turned out that the KeyIDs which I have to write into my "keymap.xml" still differ from the ones at the time when I was using KODI 17.6, so I had to determine them all once again.
But the really bad news is: now there are different buttons on the remote control where the "Keymap Editor" addon displays the same KeyID, for example button 7, 11 and 32 all are identified with "61952", button 22, 23, 24, 25 with "16839168" and so on
I don't understand how this is possible, I mean, it's the same piece of hardware as before the upgrade from KODI 17.6 to 18...
Regarding other configuration file(s) you mentioned ("remote keymap"), I would like to point out that under KODI v17.6 I only had to cope with "keymap.xml" and this way all was good to go, as KODI 17.6 actually recognized the remote control as a "keyboard" and not a "real" remote control.
I'm a bit confused: has KODI 18 dropped the support for the usage of this kind of device?
And I'm totally clueless what I would have to write into any other configuration file(s) other than "keymap..xml"...
Could you please help me a little further with it?