Hi @ian8881
If the ice build up is on the pipe coming from the condenser unit into the evaporator unit, it could be due to a lack of refrigerant i.e. a leak. Perhaps check for any oily residues around the compressor, on the condenser unit and any exposed piping. There's a special oil mixed in with the refrigerant that is used to lubricate the compressor and if there's a leak, the gas will escape undetected but the oil may leave a trace, but not always.
If the ice build up is on the pipe leading from the evaporator unit to the compressor then it could be due to a clogged filter dryer in the line
If either case, the sealed system would have to be accessed and the high and suction pressures tested to know what the problem is.
Depending on your location, you may need to get a licensed repairer to check this out, due to the environmental regulations regarding the handling of refrigerant gases, that may be in force. (where I am you can't buy refrigerant gas without a license) They also have the necessary equipment to test the pressures and also to check for a leak.
Here's the [https://www.bosch-home.ie/supportdetail/product/KGN33V60GB/01#/Tabs=section-spareparts/Togglebox=tb0303/Togglebox=tb0103/|parts list] for your model, that may be ''useful'' if it is a faulty filter dryer, that needs replacing (part ID 0103 - part # 00093110). Search online for the ''part number only'' to find suppliers that suit you best.