I have a thread which was posted in one forum, I moved it to another forum and left a re-direct. I am pretty sure the re-direct initially worked but now it gives a XF error. The main post is still accessible, no issues.
My first suggestion would be disable each add-on in turn to try and determine if it is one of those which is the cause.
This though may not be sufficient and it may even be necessary to uninstall the add-ons completely.
You may need to create a new redirect after disabling each add-on to test the functionality.
Do you have a test installation on the same server?
If not, I would recommend setting one up so you can check that redirects work and it will also give you the opportunity to install and uninstall add-ons for troubleshooting purposes.