I'm experiencing the same issue, and I apologize for not having a solution either. I've also tried searching on Google, but I haven't found any useful results.

I have a theory about why this issue is occurring. It seems that the mod hasn't been updated to be compatible with the latest game version. I thought about the possibility that "IUserMod" might be something modders need to add to their mods in order for them to work with the latest game version. It could be a requirement introduced by Paradox Interactive and Colossal Order in their latest update. That could explain why other mods, not just "Roundabout Builder," are also experiencing the same issue.

I'm not sure if my theory is correct, but it's what I believe might be the reason behind the "IUserMod" problem.

By the way; The last update of this mod according to Steam was; September 28, 2022, 2:08 p.m or 14:08 in 24 hour time.

Hey guys, I had this problem too with a few other mods as well as Roundabout Builder. I've _just_ fixed it.
1. Navigate to: \AppData\Local\Colossal Order\Cities_Skylines
2. delete it
3. restart computer
4. verify integrity of Cities: Skylines

When you next start it should load up fine and you don't get that error message.

Literally just worked for me.

Originally posted by Whiskeyjack256:

Hey guys, I had this problem too with a few other mods as well as Roundabout Builder. I've _just_ fixed it.
1. Navigate to: \AppData\Local\Colossal Order\Cities_Skylines
2. delete it
3. restart computer
4. verify integrity of Cities: Skylines

When you next start it should load up fine and you don't get that error message.

Literally just worked for me.

Hello, in step 2 "delete it", do you mean to delete the whole Cities Skylines folder?

Originally posted by JellybeanJelle11:

Originally posted by Whiskeyjack256:

Hey guys, I had this problem too with a few other mods as well as Roundabout Builder. I've _just_ fixed it.
1. Navigate to: \AppData\Local\Colossal Order\Cities_Skylines
2. delete it
3. restart computer
4. verify integrity of Cities: Skylines

When you next start it should load up fine and you don't get that error message.

Literally just worked for me.

Hello, in step 2 "delete it", do you mean to delete the whole Cities Skylines folder?

Yeah what?

Originally posted by Sherberto:

Originally posted by JellybeanJelle11:

Hello, in step 2 "delete it", do you mean to delete the whole Cities Skylines folder?

Yeah what?

Think @JellybeanJelle11 asks about it, because that deletes the hole Cities: Skylines folder. I would say as a tip, to make a backup out of that folder before you delete it. If you still want to have access to the old Cities: Skylines folder that has that error message. (Because I think that folder contains the assets and mods you have manually installed, if you play the version without Steam Workshop.)

By the way, the last time I played Cities: Skylines (1), it worked for me. I can't recall exactly what I did off the top of my head, but it worked at that time.

I haven't played Cities: Skylines (1) in a while, because I lost interest playing it since they announced Cities: Skylines 2.

Originally posted by Whiskeyjack256:

Hey guys, I had this problem too with a few other mods as well as Roundabout Builder. I've _just_ fixed it.
1. Navigate to: \AppData\Local\Colossal Order\Cities_Skylines
2. delete it
3. restart computer
4. verify integrity of Cities: Skylines

When you next start it should load up fine and you don't get that error message.

Literally just worked for me.

I had the same issue since last major update ... I just tried your trick, it works perfectly, thanks, life is nice again !

That's NOT the soloution if you play lokal ! You lost all your settings an all maps !