Youtube Android 7.1.1 -

Cause: QUIC protocol mismatches between the old OS and modern YouTube servers.
Fix: Open YouTube > Tap your profile > Settings > General > Toggle "Reduce data usage" ON. This forces the app to use TCP instead of QUIC, stabilizing the stream.

For several years, Google kept the minimum API level for YouTube low to support older devices. However, modern iterations of the YouTube app have raised the minimum requirements. youtube android 7.1.1

If the official app becomes unbearable or crashes constantly, you have excellent alternatives: Cause: QUIC protocol mismatches between the old OS