It happens frequently that some third party skins or plugins are not designed to use only the official API of b2evolution. Many are hacking into the core which creates problems on upgrade.

If you are using a custom skin, start by switching back to a stock skin that ships with b2evolution. If this solves the problem, it means your custom skin is not (yet) compatible with the new version of b2evolution you are using.

If you are using third party plugins, try disabling them all. Does it fix the problem? If yes, then re-enable all plugins one by one and test each time until you find the faulty plugin. This means that this plugin is not (yet) compatible with the new version of b2evolution you are using.