modunload fails because
$ modinfo | grep vboxflt
delivers two lines:

root@notebookcg2:~# modinfo | grep vboxflt
262 fffffffff529a000   7bd8 309   1  vboxflt (VirtualBox NetDrv 7.1.8r168469)
262 fffffffff529a000   7bd8   -   1  vboxflt (VirtualBox NetMod 7.1.8r168469)

this leads to a non working modunload option list. The mod_id's are the same, so take the last.

--- VirtualBox-7.1.10/src/VBox/Installer/solaris/vboxconfig.sh.orig
+++ VirtualBox-7.1.10/src/VBox/Installer/solaris/vboxconfig.sh
@@ -576,7 +576,7 @@
     moddesc=$2
     retry=$3
     fatal=$4
-    modid=`$BIN_MODINFO | grep "$modname " | cut -f 1 -d ' ' `
+    modid=`$BIN_MODINFO | grep "$modname " | cut -f 1 -d ' ' | tail -1`
     if test -n "$modid"; then
         $BIN_MODUNLOAD -i $modid
         if test "$?" -eq 0; then