Custom Domain Makes Site Links Inoperable

[ Moved to the Fixing WordPress sub-forum. ]

Hello,

I have a multi-site environment. The websites in my environment have domains that look like this:

https://blogs.fcps.net/mycustomdomain

One of my clients has this original domain with GoDaddy: https://www.cassidyteachers.org. It seems to have been redirected properly because when you go to that domain, it works. However, if there is a link on a post, clicking that link will make the screen turn blank.

I tried going to actual domain which is https://blogs.fcps.net/cassidyteachers. When I would click on the same link that is embedded in the same post, it works just fine.

Does anyone have any ideas as to why this is?