What makes you think it is Swathing+ causing the problem? Unless I am not understanding your issue, I just tried on my end and have no issues so possibly a different mod conflict?
I thought it was very strange too until I saw the fillType errors another user sent me and I got it to happen in my end.
Normally when you have a fillType error, the production that is missing its fillTypes just wont work. In this case, if Swathing+ is unable to load its custom fillTypes (used for rice and sorghum swathing), when my script goes to make the modifications to the fruitTypeConverter (that is what tells the game "if you see sorghum, make it sorghum swath") it trips an error and breaks ALL of the fruit type converters. Without a converter loaded to the map, the game thinks the header and harvester are not compatible (a header needs to have the FORAGEHARVESTER converter to be attachable). Side consequence, this was also breaking mowers (mowing uses a converter to go from grass crop to grass_windrow).