Mod: FS19_RealLifeNumbers_US_Heartland
I found the problem. On this map the numbering of farmlands reaches more than 200. Not that there are more than 200 fields, there is not. But the numbers used to identify farmlands runs up to 254. My mod had set the limit to 200.

In the script RealNumbersCommodities.lua locate the following lines of code:

for k = 1, 300 do
local randomFieldPriceFactor = 1 + randomness *
1 * (math.random()-0.5)/10;
RN.fieldPriceList[k] = randomFieldPriceFactor *
RN.pricePerHa; -- list of purchase prices per hectare
RN.fieldStatus[k] = " For sale";
end;

Change 200 to 300 as is done above, so that k runs from 1 to 300.

Save the script, rezip the mod and put it back into your mod folder.