Contact the developer of your plugin. They need to update their usage of getTabTitle to account for the new parameter. If they are not doing custom additions to the tab title they do not need to be including that function at all and should be extending cbTabHandler.

We require that parameter to have a value sent to it. The OPs issue is a declaration warning which won't go away by adding a default value. Adding a default value would just fix parent::getTabTitle calls from throwing an error, which we will probably add for stable release. It was going to break something regardless of what we did, but it needed to be done as tab titles had no clue where they were being output. In most cases a tab shouldn't be declaring getTabTitle at all unless they're doing custom modifications to tab titles (e.g. adding a counter like CB Articles or CB Blogs), which need to be location specific and couldn't before because $reason was never sent.

We're adding a default value to $reason for stable release, which we're planning sometime this week (probably tomorrow). This will cause a declaration warning again, but will fix fatal too few argument errors which are stopping errors. The warning shouldn't be visible if debug mode is off though so isn't a problem for production sites and isn't a stopping error.