I don't have that particular model, so hopefully someone who does will be able to help you out. Most ribbon issues I've encountered with other typewriters inevitably turned out to be either an incorrect ribbon path (from the previous owner), or a gummed up vibrator, which was restored to proper function by a thorough cleaning. However, in your case it sounds like the vibrator is lifting too high. I would begin by checking the linkages for the colour selector, and the linkages for the vibrator itself to see if anything was bent or out of place. 

Thank you both for your replies!
So I won't have to think and worry about that issue anymore ... sigh.
However, that Maritsa connection is interesting, will have a close look at my 22.

 

I think I have figured out what the reason might be for the erratic red typing of the Cellas. I have a "new" one with the same issue, another one coming in the next days, while the first one has been sold in the meantime as an all black machine. What I found:

There is a wire with a one inch loop that connects the colour switch with the mechanism that determines the ribbon lift height and links to the ribbon vibrator. The mechanism is located underneath the right spool. The angle of that wire, and the angle and position of its connecting hooks on either end, resulting in the wire not reaching its proper position in the mechanism and not being able to lift the ribbon vibrator to full extend every time aka reliable red. It is possible to push the wire manually in the proper position, and get a full red letter. Nice. Same when you manually lift the vibrator to full extend, nothing is blocked at that point. Therefore I don't think any other issues are involved.

I am considering hacking this by fixing a soft spring to the loop, in case I find a suitable spring and a good spot to fix its other end to, so that the wire gets a better angle. Hoping this will not affect the black position on the other side of the mechanism. Alternatively, I might unmount the ribbon mechanism to have better access to the wire, and rebend the ends. Hmpf.

Here you go. It comes with a shiny black plastic hood, and for my taste there is too much plastic in the mechanism.
But the name Robotron - generally only known for GDR computers, hey! - is very catchy, and so is the bright orange body design. It offers autospace and fixed tabs every 10 increments or so and is a featherweight. I'll have an off-white model soon, hopefully.

*smile* I am officing around anyway, get email alerts for that topic and was lucky to have the Cella photo at hand on my website.

Cellas often sell at less than 10 Euros on Ebay, if you find one with local pick up in Berlin, I can handle that, pack it up properly and ship it. So, basically, you needn't really consider more than the shipping costs.