kairoeder on "[Plugin: WP RSS Aggregator] Can't import feed from a w3c valid xml"

Found out that this must be a server configuration issue.
I made a 1:1 copy on another server and this still hasn't worked but then I tried a workaround I found:
- Using feedburner to fetch the feeds and then let WP RSS Aggregator fetch from feedburner
- This worked on my second server but not on the main server - even with a fresh WP installation.
- I tried a third RSS Feed Plugin which has exactly the same issue so there must be some sort of library issue? 1 worked for all feeds, 2 doesn't work (or just work on certain server)

I thought about encoding issues but I'm really not sure.
For this feed: http://feeds.feedburner.com/UkPublishingPerspectives

The debugger shows:

[17-03-2016 19:05:04] [Error] WPRSS > wprss_get_feed_items:
Failed to fetch feed "http://feeds.feedburner.com/UkPublishingPerspectives". This XML document is invalid, likely due to invalid characters. XML error: Not well-formed (invalid token) at line 1, column 1

Which makes no sense.. like 1 column 1 is:

<?xml version="1.0" encoding="UTF-8"?>