This was not the cause in my case. I did try the solution, but it didn't help. Also, the .ics worked on my laptop. I could not find any problem by inspecting all the configuration files I could find, but renaming ~/.kde and re-launching allowed everything to work again. I simply copied back everything from the old folder except the korganizer part of ~/.kde/share/apps and ~/.kde/share/config, but pointing it to the original .ics file. It has worked ever since.
Whatever went wrong, the attempt to add a second .ics was a red herring, a total coincidence. Checking in kcontrol showed that it had never been added. I have added it since then, and everything is fine.
Thanks for trying to help. I hope my report helps someone else.
Anne