I heard some months ago that not only did the US congress warn about Russia's interference on Catalonian non-official referendum, but that Russians wanted to use Catalonia as a port for their enterprises on Europe. I must say I am from Spain and I see the process almost dead. What I heard about Russians is they are not bad tourists: they learn the language quickly and they do not cause troubles on street. But as my holidays region I wonder if that malicious suspicion is grounded.
Russian state media consistently supported the right of Catalonia to secede from Spain. It supported a "Yes" vote. (source)
This is consistent with the Russian position that Crimea should have the right to secede from Ukraine. It is also consistent with a general Russian policy of supporting disruptive movements: Brexit and Trump for example.
There is little evidence of actual interference in the process of the referendum, or evidence of illegal actions by Russian agents.