It looks like 2 flight plans:
Burgas - Luxembourg
Luxembourg - Algier
Maybe some silly diplomatic stuff like Algier not accepting a plane from Bulgaria? (would require Luxembourg to put them onto the flightplan from Luxembourg, not that it really matters if it would just refuel in Luxembourg)
Overflight restrictions would apply per country i would say. So it's not that they avoided particular countries, else they would have cut through Germany on a southwest trajectory but they went straight towards Luxembourg.
Anyway i agree that it does not make much sense. Perhaps their flight planning department could not be reached or there were other problems with filing new flight plans. So they just fly the existing flight plans that previously planned a stop in Luxembourg? Seems wasteful fuelwise but i would not exclude it, sometime bureaucracy just wins. As sad as it is with language barriers and the head office in Belarus that may be closed at that hour it sounds like the most likely explanation.