Plus! didn't remove support for 8.5 (as far as we can say 8.5 still exists), but besides the Messenger servers you are also tricking Messenger Plus! into thinking you are using Messenger 2009 while your are not. This makes Plus! behave differently, they way it should if your were really using 2009, but it will fail on 8.5. Nothing you can really do about it.
Best thing is probably get over it and install 2009 for real, find a
skin you like if you don't like how the new version looks, and give it some time to get used to change.
Or you could hope Patchou puts out an update, but that would mean he needs to find another way to detect your real Messenger version just because some people start messing around with their files. I doubt the next major Plus! release would have support for 8.x versions in the first place.
/ 
