Simple enough fix. In the campaign map, under "gameParameters" there is the option to include "ranks": [ ... ]

As I'm sure you're aware, this allows you to define the amount of prestige required to rank up to each rank. If you omit this it uses the defaults from sandbox mode.

However, if you do include this you need to have all 12 ranks in there, even if maxRank is less than 12. In Iceway it only has 10 listed. If you replace it with this:

"ranks": [
{
"rankNumber": 1,
"nextRankAt": 10
},
{
"rankNumber": 2,
"nextRankAt": 100
},
{
"rankNumber": 3,
"nextRankAt": 220
},
{
"rankNumber": 4,
"nextRankAt": 460
},
{
"rankNumber": 5,
"nextRankAt": 820
},
{
"rankNumber": 6,
"nextRankAt": 1000
},
{
"rankNumber": 7,
"nextRankAt": 1250
},
{
"rankNumber": 8,
"nextRankAt": 2000
},
{
"rankNumber": 9,
"nextRankAt": 3000
},
{
"rankNumber": 10,
"nextRankAt": 9999
},
{
"rankNumber": 11,
"nextRankAt": 9999
},
{
"rankNumber": 12,
"nextRankAt": 9999
},
],

It will fix the issue.

I didn't realise, but based on what you've told me, before the AC update it seems you weren't required to have all 12 ranks listed, however, in the campaigns I always did as a matter of course so the update didn't cause an issue with the official content.

In case you're curious, the update to this part of the code with AC was to make guest entrance rates constant through all prestige values regardless of the ranks. This makes a more consistent gameplay even when we spread the ranks out more with the addition of the DLCs*.

* In vanilla you only need 6400 to hit rank 12 whereas each additional DLC ups the gaps between ranks so with both rank 12 is 9390. This ensures you have a chance to use all the new content as you rank up.

Ah, thank you for that! :) That should be very easy to fix. I have honestly no idea what's up with the error message, but I'll see what I can do here.

Great, glad I could help. Let me know if there are any more issues. :)