I have tried the following:
1. Validating the feed (it checks out)
2. Trimming whitespace and carriage returns to make the source file < 512kB (this was a longshot given Feedburner is supposed to accept files up to 1MB)
3. Changing all links to the feed and within the feed to https:// (no luck)
4. Removing redirects in the .htaccess file on the server on which the source feed is hosted (no luck, so I put them back)
I have also tried setting up a new proxy to the source feed with limited success. I have been able to set up a proxy on my personal gmail account, but not on the account that hosts the actual proxy. Once the new proxy was set up on my personal account, it also began to exhibit issues refreshing.
Any help or other ideas would be greatly appreciated!