Your comments

So, not a lot of information to go on here.

Is it just a change to http on the same proxy domain name? Or is it being redirected to http on the actual host (not proxy) domain name?  I'm assuming that your proxy is terminating the SSL and connecting to FileRun over HTTP and that your proxy addy is something like https://frproxy.external.domain and the redirect giving problems is like http://frserver.internal.domain. If this is the case, try forcing FileRun to use the proxy base by following Vlad's instructions in this answer.  If that works, then you have the same basic issue I do which is... where does FR pick up the domain name from any why isn't it detecting/using the proxy base value when generating links in content.

If the domain name on that troublesome link is actually the proxy address then you have a different issue. There are a few posts on here about forcing SSL, and I believe even a "force HTTPS" option in FileRun itself. Poke around at those a bit and see if it fixes your issue.