Spot the difference.
The former owner replaced the original SX-64 KERNAL (251104-04) with a C64 one (901227-02), most probably to be 100% compatible with all the games.
But an SX-64 doesn’t feel like an SX-64 without its unique colors. That’s why I replaced the KERNAL chip again, this time with the original one. Actually I couldn’t find the original chip (the MOS 251104-04) anywhere, so instead, I installed an EPROM chip with the original SX-64 KERNAL programmed into it.