Hello Mr. Black,
Seems like I was just confused by this statement in the documentation:
" Arrays:
In Elasticsearch, arrays do not require a dedicated field data type. Any field can contain zero or more values by default, however, all values in the array must be of the same field type." ~ extracted from (Field data types | Elasticsearch Guide [8.8] | Elastic)
However, it seems like the problem consisted of my script forwarding single elements not as arrays including only one element, but forwarding the long value as it is. So Elastic was confused to distinguish between longs and arrays of longs. I only had to make sure to use the array type at all times.
In that script, I transform the events and forward them as logs to Graylog 
Thank you very much for your help, I don’t have these mapper errors anymore! Have a great day.