The hot-fix that allowed FOSSBilling to be used when installed under a sub-folder (example.com/billing) has been reverted because it was causing issues on other installation methods.
Installing to a sub-folder rather than a sub-directory has always been discouraged, however starting with the next release of FOSSBilling it will be considered an unsupported installation method.
For 99% of you, this change is meaningless, however we do know some people have FOSSBilling installed this way.
If this effects you, you'll need to move to using a sub-domain for FOSSBilling and either re-create existing subscriptions or configure some kind of redirect for the `ipn.php` file.
If needed, we can provide some assistance with this process