Custom pages for 500 class error - can't add the page

[akaziuka]

Hello,
For some reason, I can’t add a custom page in ‘500 class errors’ area. I have validated the page and added the token as the documentation suggests but when I try to publish it, the popup message says “Your custom page must be larger than 100 characters (code 1203)” which is quite confusing. The page in not empty and viewable:
https://myabilities-err.azurewebsites.net/500.html