Yes, it's a hose run as well, it's the same problem as the OP so I didn't want to go opening a new thread unless that is required. The differences are I'm using 2017.4.2 (I didn't even know there was a newer version than 4.1 I just found out when troubleshooting this problem)
I can, however, edit old routes but can't add new routes to the old run.
I hadn't closed and re-opened the model which I find very strange, I had copied some 'conduit' in Content Center and edited the family table and properties to make a different hose. I then set up a new Tube and Pipe style to use this newly defined content center hose. Then I created a new run (let's call it Run 'B' and Run 'A' is the original run in the tube and pipe assembly) as it's in the same assembly but concerning a different part of the system. There was a fitting that was in Run A that I actually needed in the Run B so I copied this to the Run B and placed using connect fitting then I went to create a new route and that's when I got the error.
I have since tried all the ideas in this thread and a few others, totally shut down the PC, I've tried blank assemblies in different .ipj's from different templates and I still get the same error, I've tried vault content center and desktop content center. I have tried (through windows uninstall or change a program) an installation repair and a reinstall, and still no fix. I'm yet to try a complete uninstall and then fresh install, but if the below works then I think that will be my only option.
Interestingly the same file seems to work correctly in 2018 on the same machine but I don't access the same content center for 2018 as 2017 so I can't fully test it. Currently, I am working through some steps on my old machine that also has 2017 except still on 2017.4.1 to try and rule out whether there is an error in the newly added content center hose or whether it is something else.
But this would be a workaround, not a fix, it would be nice to know more detail about what that error message actually means, here is a screen grab of the message.