Hi Frambo,
If I understand your problem correctly, I think it would be worth trying a re-wire before replacing parts. There may be other problems in the mix as well, so this is what I would suggest, in order:
1) Most generator-lighting problems are related to wiring and poor grounds. Both can be solved most handily by using double-lead wires throughout. Using the bike as a ground-return doesn't always work very well because the ground can "float" and prove unreliable over time. For example, it is a long ways from the fork crown to the taillight, and with bike-earthing, connection is also made via the headset bearings, which can be really problematic in practice.
Because of this, I would suggest first running new double-leads from the Shimano dynohub to the headlight, and again from the headlight to the present taillight. If it is a single-wire taillight, then one lead will replace the present one, and the second lead will be grounded to the mounting bracket for the taillight, providing the shortest possible path for ground-return. Basically, electricity will go down one wire and back through the other, and the bike will be left out of the equation.
I presume your taillight gets its power from the headlight; is this correct? If the headlight provides the power and the unused headlight ground lead (normally intended to run to a two-taillight) accidentally grounds against something like a brake caliper, it can lead to some of the problems you describe. It would pay to check this.
2) I am not sure which headlight you are using, as there are several in the B&M line that are called "Lumotec Senso". From your description, I think you may have some version of the Lumotec Cyo (an LED headlight). If that is the case, you probably do not need the voltage limiter, as that task will be handled internally by the headlight.
If you have an incandescent headlight and taillight, then I would place the voltage limiter between the dynohub and the headlight, and not between the headlight and taillight. If the voltage limiter is not needed, then it should be removed.
3) If neither 1) or 2) above fails to get everything working, then there is something else going on. For example, the sensor circuit inside the headlight may have failed, or the switch on the headlight may be faulty. This is not uncommon with some earlier models of IQ Cyo Senso headlights. Unfortunately, they are not readily repairable but can be replaced.
4) If you need a taillight, then a very good one is the B&M Toplight Line Plus, which uses two LEDs and a prism to spread the light in a band across the entire taillight lens. It is bright and noticeable without being obnoxious or irritating, and the larger lit area helps closing drivers to better judge their distance to you.
I hope this helps, Frambo. This is how I would approach the problem, and the process nearly always brings success.
All the best,
Dan.
