This is compatible issue between CS1308 and Mac mini that the old ASIC:8021 chip can not support new Thunderbolt I/O, it can be solved by new ASIC :8021A now.

If you are unable to get a new CS1308 to solve your problem, below is an alternative Solution: 

1. Instead of Apple original mini Display to VGA adapter, use ATEN  mini Display to VGA adapter VC920. 

    Note: If use VC920 connect the CS1308 and Mac mini,  the max resolution is 1280x1024, and can't see the booting process of Mac mini. 

2. Please use other KVM switch which does not use ASIC:8021 chip