Automatic Custom Columns with YTD Calculation

There is no way for you to attach a file directly. Generally a shared cloud storage location that is accessable to a public user would be the only way to share a file. 
Specific to the code, it may be an issue with date formating. 

Try this version to see if it removes the errors...

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("dc5RDoAgCADQu/DtJiIid3F+1P0PEdkqavoBg/EGtAYbBEiULY/KAiliioTE10gZepjBVDwswjLgbh0X8TA7SOds7qg6J7ebHDZHr1sy9axqfc7+HH3XsS4gZgd1/NcP", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [CATEGORY = _t, SUPPLIER = _t, CLASS = _t, AGGREGATE = _t, MONTH = _t, VALUE = _t]),
    #"Parsed Date" = 
    Table.TransformColumns(
        Source,
        {
            {"MONTH", each Date.From(DateTimeZone.From(_, "de-DE")), type date}
        }
    ),
    #"Changed Type" = 
    Table.TransformColumnTypes(
        #"Parsed Date",
        {
            {"CATEGORY", type text}, 
            {"SUPPLIER", Int64.Type}, 
            {"CLASS", type text}, 
            {"AGGREGATE", Int64.Type}, 
            {"MONTH", type date}, 
            {"VALUE", Int64.Type}
        }
    ),
    #"Grouped Rows" = 
    Table.Group(
        #"Changed Type", 
        {"CATEGORY", "SUPPLIER", "CLASS", "AGGREGATE"}, 
        {
            {"January_YTD", each List.Sum(Table.SelectRows(_, each Date.Month([MONTH]) <= 1)[VALUE]), type nullable number},
            {"February_YTD", each List.Sum(Table.SelectRows(_, each Date.Month([MONTH]) <= 2)[VALUE]), type nullable number},
            {"allRows", each _, type table [CATEGORY=nullable text, SUPPPLIER=nullable number, CLASS=nullable text, AGGREGATE=nullable number, MONTH=nullable date, VALUE=nullable number]}
        }
    ),
    #"Expanded allRows" = 
    Table.ExpandTableColumn(
        #"Grouped Rows", 
        "allRows", 
        {"MONTH", "VALUE"}, 
        {"MONTH", "VALUE"}
    )
in
    #"Expanded allRows"