I was having the same error:
XML Parsing Error: XML or text declaration not at start of entity
so I followed all your suggestions (clear blank lines, disable plugins, etc) and it still wasn’t working.
The feed was working fine before I changed my permalinks to a Custom Structure (/show/%postname%/) so I figured it had something to do with that.
The feed produces an error at the feed-URL generated by WordPress.
The feed works fine if I add the feed parameters to the URL.
So:
https://beetik.com/feed/ (doesnt work)
https://beetik.com/feed/?feed=rss2 (works !)
I’m using that quick-fix as a resource… but I had to hard code it, so I hope there’s a better fix in the long run.
