The option jjreds pointed out worked. Yesterday when I constantly had the problem on POIloader I selected the exact directory where all *. bmp and *.gpx files were located. For example C:\GPXfiles\Oregon\Default\.
This resulted in not having the "All categories" option.
Now I selected NOT the folder where the aforementioned files were located, but one folder higher, being C:\GPXfiles\Oregon\. This did the trick, the custom POI database now shows a button "Default" containing all the custom poi's. It's as simple as that. Thanks jjreds.
Now to answer your question, I was using version 2.80, in which I didn't have any problems, there was no splitup in Traditionals, Multi, etc. available in that version. Or if it was, I never saw it. Then I upgraded to 2.85 and the problem I posted occurred.
Today I upgraded to 2.86. Obviously Garmin realised the "All categories" option was handy after all and they therefore reinstated it I guess. However I get an error message when I select this "All categories" option. If I interpret your post correctly and by reading the various threads about 2.86 I guess I am not the only one who has this problem, it is off course still a Beta version.
I am drifting a little off topic here now, thanks everyone for the help, my POI problem is fixed.