ᐅ The Vaillant VCW 196EC is experiencing intermittent operation issues.

Valves on the return line... where can I find them? I’m not a professional...
The installer who didn’t install the heating system said years ago that it was oversized for an apartment of just under 100 m² (1,076 sq ft).
Also, some sensors with wires on the top left were replaced at one point.
Then, during the switching to H-gas, the technician mentioned that most professionals don’t dare to adjust the flow and return times.
That just came to my mind.
I also personally bleed the system according to the rules at the start of the heating season: turn off the heating or pump, set all thermostats to 5, wait 30 minutes, use a special radiator key, then check the display—maximum 1.5 in the green zone. The hose of course should be filled with water beforehand...

Perhaps your system still has the older pipe layout with the valves hidden somewhere at the bottom of the riser?

Definitely more than 25 years old... Do they have a valve? Or are they located inside?

For buildings older than 25 years, you usually have internal return valves rather than visible shut-off valves. These are located directly at the radiator connection and look like a small screw or an Allen key. If one of these is partially stuck, the flow rate will drop significantly, and the Vcw will report exactly these errors...

Okay, is there one of those parts on every radiator?
And what should I do with it without causing a flood before the weekend?

Each heating circuit has a valve cartridge installed. It looks almost identical to the valve fitting. As long as you only move it slightly, you don’t need to drain any water because the system is sealed and pressure-tight. Sometimes, just a small turn is enough to get a stuck cartridge moving again. First, check a single return line after a few minutes of heating operation to see if it feels noticeably cooler. This way, you can quickly tell if the flow rate is being restricted somewhere.