I've come across mistakes and out-of-date information even in the most reliable guides. When two trustworthy sources differ, you might have to go to the source to verify. The number for the albergue is on Antonio Retamosa's site:
Camino de Santiago por la Vía de la Plata y Sanabrés. Todo lo que necesitas saber para tu Camino: Etapas / Mapas / Albergues / Consejos
viadelaplataelcamino.blogspot.com
Whatever information you get today, the situation might change between now and February if a pipe bursts or someone's health fails. If you stop at Tabara on your way to Santa Marta, you can be sure that the hospitalero there (Jose Almeida) will offer you the most up-to-date information and advice about the next stage.
In the event that Santa Marta is closed, you could divert to Camarazana de Tera, where there's a hotel or you could continue along the Camino to Calzadilla de Tera, which makes for a somewhat long stage. Fortunately the path from Santa Marta to Calzadilla de Tera is quite gentle and flat. (Unfortunately, the walk from Tabara to Santa Marta is quite strenuous). After that gentle walk, you might feel like continuing another couple of kilometers as far as the albergue La Trucha in Olleros. It is run by a delightful old lady who serves home-made sausage in her bar, but the single-walled cinderblock dormitory is as cold as the North Pole.