This is a known, memory-related issue with Chromecast 1st & 2nd generation. When you play content at too high a bitrate, the Chromecast's Default Media Receiver sometimes can run out of memory available to it, which can either result in freezing or poor playback experience (which may manifest as buffering).

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In this cases, you can fix this problem by following steps below.

1. Disconnect your Chromecast with the app.

2. Open the app's Preferences Settings, and choose "Medium Quality" option in "Video Quality" section.

3. Reconnect your Chromecast with the app and stream that video again. 

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Video streaming should be working alright again.