Stuck import – New posts are not being imported

I am running into a very peculiar issue with one of my feeds. I have several xml feeds (same variables/attributes on all imports), same size 1mb, 400 posts.
I have not made any changes in the configuration in the last couple of weeks, my environment is hosted on my own physical servers.
I have a cron job that runs an overnight import. All imports went fine, except a single feed. The feed that failed, stuck at importing 80 something new posts of out of four hundred.
I have attempted to re-start the import manually, however the import failed with the following message – “Your server the imported process”.
If I use Trigger / Processing script URL, the following scenario occurs, every time the processing URL is run, I get the output of the log with the posts which are being imported. The page always displays the same exact posts. Every 20 to 30 minutes, while trying to run “processing script url” I am able to import 20 or so posts (the default set under import settings – “each increment process XX records”).
I have checked php5-fpm logs, access / error logs from my webserver, sql errors and I am not able to find a single error message that would point me in any direction.
I was using an WP ALL Import 4.3.0 Pro, after the issue arose, upgraded to 4.3.1, the issue was not fixed.
I re-created the feed, same scenario repeats itself over and over again.
If the import is re-run, it runs through all of the existing posts without any issues, but fails while trying to import NEW posts.
My config settings are below, if you see anything that stands out, let me know.

PHP Upload Max Size: 512M
PHP Post Max Size: 512M
PHP Upload Max Filesize: 512M
PHP Time Limit: 600
PHP Max Input Vars: 10000

Any assistance will be highly appreciated.