Yes you are right.
Now it is looking for the needle in a haystack (a German proverb).
Please disable all AddOns (except TongassFjordX).
Continue disabling all additional object libraries (including RTMM).
Now only the P3D standard sceneries, TongassFjordX and SIRP should be active.
Please leave the 00_P3D4_SIRP_Buildings_LIB.bgl deactivated.
As a result, a lot more objects are missing in the SIRP base ...
But it's only about the black buildings.
What does the result look like now?