I have pinned this one down. In relation to your save and picture there are two aspects:
1) An outright bug in the caching (which I will fix and push to SVN as well as the patch thread later today)
2) Great farmers can only place bonuses on tiles that:
2.1) Do not already have a resource (INCLUDING ones you may not yet have the tech to see, so sometimes actions will be missing because of a hidden resource)
2.2) Are part of your trade network (i.e. - have a route on them connecting it to a bonus of the type needed for whatever you want to place)
The plot you are trying in the image fails on account of (2.2) above, as well as (1), so even after I fix the bug, you will first need to build a road there to get the great farmer actions enabled for that plot.
Okay, i have now tested a few things to provide yo with informations:
I have changed the iTrain value from 750 to 749 (a harmless change, only to cause the recalc-popup, because i can't remember the keys).
This is, what happened:
The recalc option appears, and i have choosen "yes". After the recalc i was able to use the great farmer correct. Now i can place Ressources according to the rules for.
Also my netto gold income grows from about 1.700 to about 5.000 Gold.
Now the recalc option appears every time, when i load the game, although i made no new changes. When i choose "yes", then the gerat farmer break his work and i was no longer able to set ressoruces. Also the gold income shrinks.
When i choose "no", then i can play normally.
I hope, this information are useful.
EDIT:
I have undo the change (749 --> 750), and after the recalculation the farmer works normally. By reloading the game, the recalc option no longer appears. My brother means, perhaps is the value 750 (in this exampel) in the savegame enshrined, so the recalc option appears every times, when a other value in the xml contains. Then it dosn't matter, if i made a new change, or not, then the option appears always.
EDIT2:
Strange... After a total restart from the game, the recalc option appears again...
