Please say which language you're using. I guess it's C# but I should not have to guess, and I cannot be sure. Make things easier for people inclined to help you.

Re-raised by OP above.

Please see language reference, section on "Iterator validity".

Do you know what iterator invalidation is? After calling insert, iterator j may become invalid and does not point to a valid position in the string anymore. Further attempts to use it cause crashes.

Yes, you can: https://docs.codewars.com/training/troubleshooting#post-discourse

It means that your code has a bug and causes solution to crash. It's difficult to tell exactly why without seeing your code, but some ideas can be found here: https://docs.codewars.com/training/troubleshooting#appendix-errorcodes