From what I have read, these are very obsolete systems that are useful primarily because of their long range and the fact they exist (the most useful weapon is the one you have).
Ukraine has been known to use these as part of SEAD/DEAD operations. You launch one of these easy-to-intercept missiles at a high-value target. Russia is then forced to engage with their air-defense assets, which reveals their location. Nearby stationed Ukrainian aircraft simultaneously launch HARM missiles which seek out the now active air defenses.
Cool to see a Korolev Cross on something so small. Korolev was Ukrainian. He was the "Werner Von Braun" of the Russian space program. The Soyuz rocket, which even today, is the primary spaceflight rocket of Russia was Korolev's design.
My guess is: incomplete combustion. As in some rocket propellant exited the nozzle of the rocket without burning, and then burning or just expanding in atmosphere (instead of under the high pressure engine conditions) where it lacked enough oxygen to burn white like the rest of the exhaust you see.
In short: old rocket shows its age with a slight hiccup. Its not a big deal because this will be its one and only flight.