B2. A previous installation worked fine, but the latest version causes problems.

Sometimes this problem occurs if a new version is installed over an old one. It is strongly recommended to always install new versions from scratch into a new directory rather than overwriting an existing installation. Installing from scratch can also solve problems caused by addons which are no longer compatible with a new version. If you suspect a problem may be caused by an addon, re-install your addons one at a time and see whether you can identify the culprit. If you can, you may consider notifying the author of the addon.
Multiple Orbiter installations can co-exist on the same computer, so it is a good idea to keep your old Orbiter version to fall back on, in case you can't get the latest version to work on your system.