After reading through most of the posts, it's surprising that QFE/QNH problem is mentioned only a few times and then quickly forgotten.
Even on the international airports in Russia ATC insist on QFE altitudes, and they are in meters. QNH is available on ATIS or on request but clearances are issued based on QFE altitudes. For crews not regularly flying in Russia it can all be confusing, especially if using NAV charts in feet based on QNH, with QFE altitudes in brackets and conversion tables to meters. And higher elevation airports have large difference between QNH and QFE which can translate into high altitude errors. Large enough that legal minimum by one is 10 ft under ground by the other.
As it is AF base, we don't know what type of charts were available. May be some nonstandard (read not normally used by the crew) type based on QFE and/or just meters. Add ATC communication in Russian with which the crew was not fully familiar, low visibility, ATC suggesting diversion and VIPs breathing down their necks and you may get some idea under what kind of pressure the crew was.
At the end of 80s Yugoslav AF AN-12 cargo plane crashed in Yerevan under similar conditions while flying aid for earthquake victims. The crew was used to QNH/feet while ATC used QFE/meters. Airport, which was at normal times just a local thing, was overloaded with aid flights from all over the world. ATC was speaking only Russian and hardly able to cope with the traffic. The aircraft crashed 2-3 km short of the runway.
Smolensk North airport is XUBS, elevation 820ft