Update: I went to the local Apple Store and the person who helped me was able to solve the issue by just disconnecting and reconnecting fans and battery (and free of charge!). He said the connectors were securely in place, so maybe by unplugging the battery something got reset in the circuit (something that had not been fixed by SMC/PRAM reset). @hellomacos your original advice turned out to be the solution! Thanks you all :)