This is certainly not a common issue, therefore it is difficult for me to diagnose. Based on what you are saying, I think I might have an idea but I need some more information. 

First, what is the link (you can exclude domain name) that you are seeing for the Forum Manager? It should show something like .../ctl/ForumManage/mid/xxx/Default.aspx. 

Second, what is the link you are actually being sent to (ie. what is the browser link AFTER the redirection)?

My guess is that the module action (ForumManager) is not setup properly with the module definition (available via Host -> Module Definitions, edit Forum module). 

How many instances of the module do you have on the install (and specifically, how many in that portal?). Are you using any copies of modules, or are any in the Recycle Bin? 

Here is an update:

I added a new forum module to my page and moved all of the forums to the new groupid for that new forum module. I also split out all of the private forums that were in that forum module and put them in another forum module on a new page. So I have forum "A", which is a collection of my public forums on one page, and forum "B", which is a collection of private forums on a separate page.

When I went to check today, I again found that the Forum Manager link had been changed in forum "A", but not in forum "B". There were new posts in forum "A" and there were no new posts in forum "B"

Here are the urls from forum "A"

This is the link to the forum "A" admin: /Forums/tabid/126/ctl/ACP/mid/896/Default.aspx

And this is the link for the Forum Manager in forum "A": /Home/tabid/59/ctl/ForumManage/mid/896/Default.aspx

And when you click the link, it goes here: /Home/tabid/59/ctl/ForumManage/mid/896/Default.aspx The breadcrumb shows that I am on the Home page, but the Home page is not displayed.

I hope this helps with troubleshooting. If there is anything else I can do to help resolve this, let me know.