V2.9.1 is a patch release that address a few small issues that coincidentally were all found on the very same day that we released V2.9.0 (darn you Murphy and your law!)
First, we found a small reversion in the line search. Through some refactoring, one of the default settings for the type of bounds enforcement in the ArmijoGoldstein line search accidentally changed, and we patched it back to the original default. So be warned, that if you updated to 2.9.0 and are using that line search you might see some weird behavior. If you go to 2.9.1 though, it should be all good.
The other issues are really minor and probably not stuff regular users will run into. You can see all the details in the release notes.