Thanks for your quick answer, Sasa! The inner side of the PC is rather clean from dust, so I don’t think that this is the problem. Before disassembling the display I would rather check whether the panel components work as they should but I have no specifications - do you?
The problem is that the PC is not usable with an external display after the internal display is deconnected, at least it doesn’t boot anymore. That’s all I know. But I still need to have access to it from time to time. That’s why I won’t disassemble it before it is inevitable.