Ipad 9th Generation can't open some links

This is unrelated to your iPad.

If I manually edit the URL to switch \ (encoded as %5C) to /, the webpage loads fine.

It appears somebody or some app involved here somewhere made an error in the URL syntax.

Microsoft Internet Explorer (long outdated, and finally retired) treats forward and backward slash as identical, IIRC.

The RFCs and many other web browsers all assume forward slash.

Contact the website maintainers.

My guess is that this is some old Windows app running on some old Windows box, and it’s basically broken.

Helals114 wrote:

I found online that the "%5c" should return "/" by the service.

Nope.

”%5c” actually represents a backslash. Its presence suggests some possible website security issues.

Here’s some reading.

https://security.stackexchange.com/questions/162940/function-of-5c-in-a-path-traversal#:~:text=%22%5C%22%20(encoded%20backslash,may%20work%20%2D%20allowing%20the%20attack.