Hi Bartl,
It may be a temporary server problem. Would you please confirm if the issue occurs again now?
Actually, a runtime error is a software or hardware problem that prevents a program from working correctly. For more information, you can check the article below:
What is a runtime error?
http://windows.microsoft.com/en-gb/windows/what-is-runtime-error#1TC=windows-7.
For the situation, if the issue persists after some time, to further narrow down the cause of the issue, we can take actions as below:
1. Use different browsers to make these changes in the Office 365 Admin portal.
2. Check if the Runtime error is specific to this particular site.
3. Recall if there are any changes made to browsers recently.
4. Check if the Windows Updates are up to date.
For any updates, please let me know when you have time.
Thanks,
Ida Qiu