It’s a file size limit, 800 kB, so splitting posts won’t do anything, I’m afraid. The feed have to contain less content (fewer bytes). I think the problem will go away if you choose to paginate the category pages by going to the Design page and checking the box next to Paginate category pages.
The feed size limit is an arbitrary one @manton has decided on for Micro.blog. Other feed readers, like Reeder and DEVONthink, might have other size limits or no limits at all.
Regarding the Twitter error: what the shortcodes does is looking up information about the tweet when building your website. You probably have a shortcode somewhere including a tweet that no longer exists, which throws an error from the Twitter server. If you can identify this post and remove the shortcode, the error should go away.
I know that other Micro.blog and Hugo users have stopped using the Twitter shortcode for exactly this reasons. Tweets disappearing is common enough, and they don’t want their sites to break every time that happens.