Only time I have ever seen this error, is when I have exceeded the max number of items, it only pops up when attempting to render and only when you either stop the render or it finishes. I would then close that dialog, check the stats as suggested, and adjust the max number of items for that forest object and on re-rendering, it usually cures the problem..
Also when you say you cannot get to change the number, because this popup keeps appearing. It should only appear AFTER hitting render and then only once the render completes or you hit stop.
Since the error message isn't a bug, I have a feeling something is very wrong
I have never had this popup appear and then been unable to continue.. BUT I am going to do a test now to see if it happens with the latest version of FP. It's fairly easy to get it to pop up. I'l report back whatever.
Please note: You have to make sure you increase the max number sufficiently, or this error will keep popping up, until you do.
OK, I tried this with a huge 30m x 30m plane, using forest pack preset detailed version, just to push it, set to FULL distribution. NO camera clipping.. I got the error message. I then adjusted the RENDER "Max Items" as below, from 500000 to 5000000 (added a zero) which was probably a bit more than necessary, but just wanted to be sure it would render. On second attempt to render it worked just fine
Oh you have to adjust RENDER Max Items value, not the Viewport Max Items value.