Why does one of my HiFi speakers only start working after several minutes?
no, I just checked and they are fine. Headphones also disable the speaker-out on this model, probably that's obvious


Ok, here's the service manual with circuit diagrams. Power amps at pages 39-40 originate audio, forking to SPL/SPR for speakers, and HPL/HPR for headphones. As headphones sound ok for you, the issue is somewhere further that circuit: connector I, relay RY501 and its driving circuit built on Q501-Q503. However as you said one channel works properly, and another with delay, I would question the relay itself first and its contacts. If relay driving circuit would be an issue then both channels would behave the same. Probably relay gets warm as it starts operating properly with one of its contact pair. Relay part number is 1-515-920-11, not sure if it is serviceable, but you may be able to gently tap it when there's no sound in one channel to see if it changes the situation. Also not clear if relay's contacts are normally open (however it is stated so in circuit and on the PCB in the manual).