If anyone have trouble with Sidekickv3 on a clean system (it won’t load and Renoise and other hosts gives you an error) - here’s the solution: It needs “msvcr71.dll” in it’s windows/system-folder. Took me some trial and error (and examinating the dll-file in notepad) to figure that one out.
Another note from the future. 
I tried to open old Renoise songs where I used Sidekickv3 for sidechaining, and only one instance of the VST-plugin could be opened at the time (you connect two Sidekickv3 to create sidechain), so in other words - it didn’t work.
I guess the reason is that the VST-plugin is 32 bit and when you run it in 64-bit Renoise, they are bridged - which somehow breaks the plugin. I tried installing the latest 32 bit version of Renoise and then Sidekickv3 still worked. Perhaps more recent versions (I saw that it is become commercial these days) will work fine though.
That said, the better long-term solution is to replace Sidekickv3 with a native Signal Follower (on one channel) and a Gainer (on another channel) and do sidechain this way instead. 