My 1964 Kramers' Engels dictionary shows "allesbehalve" as one word not two, with the translation being "anything but". Are both versions correct?
Yes. They are basically interchangeable.
Note that you should be able to hear the difference (you should hear a pause when it's two words).
Mijn Nederlands is misschien alles behalve goed nu, maar het wordt elke dag beter ;)
Duolingo, encouraging you to quit learning languages since 2011.