Which version of LR are you running exactly (use the System Info in LR to get this information) ?
Try again to reset the preferences: http://www.lightroomqueen.com/articles-page/how-do-i-reset-lightrooms-preferences/
If you are on 2015.2 or 2015.2.1 deactivate the new import:
- Go to Lightroom > Preferences.
- Click on the General tab
- Uncheck “Show ‘Add Photos’ Screen”
- Restart Lightroom
or even better roll back to 2015.1.1: https://helpx.adobe.com/lightroom/kb/roll-back-to-prior-update.html
I am having the same issue. I can only export one file at a time. I have been working with this problem since the CC 2015.2.1 upgrade. Several others on the forum are having the same problems. Up to this point, none of Adobe's solutions have helped. I spent over a half an hour working with a tech person on the phone. he had no answers, and he would not let me talk with anyone else. We need Adobe to read the forum and address this.
Last night I was desperate, and while waiting for someone to reply to this, I started searching other threads, and found the ones you linked to.
I had tried unchecking the "Show 'Add Photos' Screen", and turning off GPU acceleration.....but nothing worked.
I ended up rolling back to 2015.1.1 and life is good now.
You may end up having to roll back to 2015.1.1 just like I did.
I like it better anyway, because I get my import options back.