Weird
One thought would be to extract the AC81 pla library file so you create an actual folder with the Library objects in it. Just to double check could you post a screenshot of your Library Manager settings just to make sure you have got the libararies loaded ok. I've added a pic of mine to compare with.
Other thought would be to uninstall ArchiCAD and reinstall it. That seems a bit over the top but may sort out whatever is going on.
AC versions 3.41 to 25 (UKI Full 5005).
Using AC25 5005 UKI FULL
Mac OSX 10.15.7 (19G2021) Mac Pro-2013 32gbRam AMD FirePro D500 3072 MB graphics