These are likely two different issues. "Offline" just means that the videos you imported into the project have either been moved or renamed since the last time the project was opened, so EffectsLab needs you to tell it where to find them now. You can right-click on the thumbnails in the Media tab of the Toolbox to reconnect those clips. This knowledgebase article
explains the process in more detail.
As far as the black clips, did the footage come from iMovie? There has been a recent development with the Apple Intermediate codec used by iMovie which no longer allows it to be used in VisionLab. We aren't sure exactly what Apple changed, but something made that particular codec abruptly stop working. The solution is just to export the footage from iMovie to a different codec, I recommend Animation, using the "Export to Quicktime" option in the Share menu.
If you aren't using iMovie, let us know, and we will look into an alternate solution for your issue.