I've had a new Multisite installation that had been working

I've had a new Multisite installation that had been working fine.

Yesterday I started using Batch Create to upload all the sites I needed to create for our users. I needed to Batch Create 3600 new sites

Ever since doing that I have a 500 internal server error when trying to log in to wp-admin.

The sites load fine for visitors, but users can not log in to wp-admin because of the 500 internal server error.

I've tried:
Renaming plugins folder -- did not help
checking the wp-config file -- it seems okay
checking the .htaccess file -- it seems okay.

I'm stuck and don't know what to do.