What went wrong during development?

Those marks run the long way (assuming it's 35mm, which would fit with your 300 ml developer volume). That pretty much means they got there when you hung the film to dry, which means they're most likely due to mismeasuring your wetting agent for the final rinse. Too much wetting agent can leave a residue that dries on the film, rather than running off cleanly, and the resulting marks will look much like what you see there.

Fortunately, wetting agent residue is usually easy to fix. Just rewash the film, and treat it with the correct ratio of wetting agent to water before hanging it back up to dry.

In my experience, fixer doesn't leave flow-like marks, and if your film had touched between turns on the reel, you'd have huge blotches of undeveloped and unfixed halide visible where the liquids were kept away from the emulsion. Fixer that's only its very last legs can leave density variations, but they'll usually be more like developer flow marks, which would be vertical in the tank orientation (not aligned along the film).

I think the artifacts are too vivid to be residue from the rinse agent. If re-washing and re-rinse agent did the trick then OK. If not - re-fix twice normal fix time and wash and rinse. I think the film is properly fixed. If I am right, then this will do the trick.

It does look somewhat like the kind of marks you can get when the film is not perfectly wound on the reel. Often you would get completely undeveloped bands running across the width of the film, but you can also get areas that developed incompletely. Any kink or overlap on the reel causes this.

But there could certainly be other causes, from improper developer dilution, temperature, incomplete rinsing, no agitation, light fogging, and so on.