Why did the speed of this plane drop so much?
At that point, altitude shows about 11 km, or FL330. Up there, 380 km/h, or 205 knots true corresponds to something like 124 knots indicated, well below clean stall speed.

This is ground speed though, so with very strong head-wind it could be possible. But the only sufficiently strong wind is the Jet stream and flying eastward they would have it in their backs. 

Also for westbound flight, the dispatcher would have to make rather big mistake to send them across the jet stream—they do take forecast winds aloft into account when preparing the flight plan.

Data error is the only plausible explanation here.