1.
The problem is not if the key exists or not.
you can write

$ffx = DelTree("HKEY_LOCAL_MACHINE\Software\Mozilla\blabla")
exit @error

and save it to "testbug.kix" too.

When opening under 4.53 all works, in 4.60 there comes:
"ERROR : failed to find/open script [testbug.kix]!"

(Tested under Windows 2000)

2.
You are right with your possible solution:
when I change
exit @error
to
exit @serror
then it works in 4.53 and in 4.60 too.

3.
The problem is, that if I want to use 4.60 I have to change the scripts, because some commands could effect some things like the described.

4.
Result:
Commands which have no problems in 4.53, now could have problems in 4.60.
Thats a risk which it is not necessary to have.