“Video output is currently active. Please turn off any outputs to change video settings” is one of the most irritating error messages because it barely gives you pointers on how to fix the problem. It means that your OBS program is currently recording or streaming.
B-frame is short for bi-directional predictive frame, a form of video compression. In the 1800 frames of your one-minute video, you are the only moving object. The wall remains still and unchangeable. To cut down on the file size of your video, it is compressed. That is, only the pixels that change position from frame to frame are retained. B-frames perform compression by consulting the frames that come both before and after a frame. So if you have frames 1, 2, and 3, in order to render frame 2, a B-frame checks the pixel alignment on frames one and three. If the pixel alignment is different, then the changed pixels are the only ones that are stored on frame two and later rendered.
A common problem cited by many OBS Studio users is its RAM usage. As a video processing program, OBS Studio requires a good chunk of RAM to function properly. Since users might be livestreaming something like a video game, which also needs a lot of RAM to run properly, a RAM overload is bound to happen on a computer with low RAM or many programs running simultaneously.