toiletvalve
Suddenly the cistern is only filling after a delay from 2 to 10 seconds whereas it was immediate some days ago. I didn't touch anything and I'm sure noone entered my flat.
Not sure if it is a problem though as there isn't any leaking.
I don't know if it is linked but it seems this
doesn't serve any purpose since the cistern is filling whatever if it is open or closed.
Here is the best picture I could take:
It seems as if you have an obstruction that may be floating around in the pipe or valve. Sometimes it obstructs sometimes it flows clear. I would start with changing the water valve. If this is the old style "gate" valve there are several things that can clog the line. I would replace it with a 1/4 turn ball valve. That will give you much better flow than a gate valve. You will need to shut down the water probably at the main. The old valve is probably soldered, but you could cut it off and use a compression type valve instead. "Sharkbite" is one such supplier. Although I have never used their compression valve, I have used their end caps and found them to work well.
If you can have someone hold a bucket under the pipe once you cut off the valve and then a second person turns on the water main to fill the bucket to purge the line of any debris. With cast iron pipes it is common to get clogs from rust build up. The rust may not pass the small opening in the old gate valve when you first purged the line.. Although you did not mention what type of pipes you have, if it's copper than you won't normally find rust obstructions..
It's definitely a backup from a source higher up than the toilet. It could still be on the same floor of the house/building. i.e. The washing machine drainpipe is likely higher than the toilet.
The backup can be clear if the source is something like a shower, dishwasher, washing machine.
Since you are in an apartment, you should first contact your landlord before trying to clear the blockage yourself.
Best Answer
That valve is necessary for whenever you need to service the plumbing inside of the toilet, it turns the water supply off. In the cistern on the other end of that pipe coming from the valve, there is a "fill valve" mechanism of some sort, tied to a float. When the water drops down into the bowl to flush, the float drops and enables the fill valve to refill the cistern, until the level raises again and it shuts off.
Most likely your fill valve is getting old and is sticking when the level drops, so it shows up as a delay in that the float is not falling when it should. This would be an indicator that it is close to failing completely and needs to be fixed / replaced.