Comment # 40 on bug 1178360 from Stefan Hundhammer

A complete shot in the dark (and, as noted before, I know nothing about kernel
stuff and how all that works):

https://build.opensuse.org/package/view_file/openSUSE:Factory/virtualbox/fixes_for_5.9.patch?expand=1


Index: VirtualBox-6.1.14/src/VBox/Additions/linux/drm/vbox_ttm.c
===================================================================
--- VirtualBox-6.1.14.orig/src/VBox/Additions/linux/drm/vbox_ttm.c
+++ VirtualBox-6.1.14/src/VBox/Additions/linux/drm/vbox_ttm.c
@@ -445,7 +445,11 @@ err_free_vboxbo:

 static inline u64 vbox_bo_gpu_offset(struct vbox_bo *bo)
 {
+#if RTLNX_VER_MAX(5, 9, 0)
     return bo->bo.offset;
+#else
+    return bo->offset;
+#endif
 }


This affects an offset, and the function name suggests it's related to the GPU.
That might be a hint.