Answer in this other topic, so I copy here:

"Might be the issue there. Until the latest patch Chalk Canyon could cause vertex errrors, but the rest of the tracks should mostly be fine. As I have a medium-ish computer I play with grass on high and not ultra, and I try to reduce the areas of grass to prevent this, but might be a bit too much on higher resolutions.
Are anyone else experience this?

Hyp: If you reduce the grass settings one level - does it help?"

Sorry for late reply, Steam does not notify me when people post / create new topics. These new topics are not put on top of list either (the 3. topic view on fronpage)... stupid Steam :C

Alright I'll probably try lowering the grass. Although I have it on Ultra and it runs fine on official tracks. Thanks for the reply. Some great tracks in this pack by the way, some of that craziness that is maybe lacking from official tracks, gives nice variety

(And sorry for 2 threads, wasn't really sure where to post)

Thank you :D I guess it's just a matter of BB being a little better at managing the resource usage and grass usage, I am still not very great at creating too advanced clutter masks (which areas to be covered by grass and whos not), meaning it's a bit of trial and error. BB mentioned that they probably / maybe will make it so that the game limits the vertex to what it can handles / a max, meaning we / I won't have to update the excisting stuff if the implement it. If this really is going to happend or not I am unsure of, but for now try running on high and not ultra when on tripple screens.

On top of my head I think Kristiansand, Arendal Triangle, Pine Hills Raceway, Black Sails Valley, Chalk Canyon and the upcoming Sca Ultimate Speedway are tracks that all features pretty (but demanding grass), which might be a issue at tripple monitors.

Kristiansand was erroring for me, lowered the grass and worked fine. So yeah, your on the money with what the issue is :)

Btw, someone said in a other topic that running mixed or multiclass AI sets can help that trigger often too. What AI sets you use?

I may have used a custom AI set with all vehicles (ABC + harvesters, bus etc). But I have no problems with this AI set on official tracks, even Ultra grass enabled

Lets take that back, I have also Out of vertex memory on official track (Espedalen) if using the new official "Mixed" AI set. Maybe this is something BB needs to fix. I guess the additional car models take more VRAM, and then if graphic detail is high, something goes wrong

By lowering grass setting, I can run all tracks, all AI sets

I just came across this problem today. I kept the grass on ultra, but changed the AI from mixed to banger. That worked for me.