Failed Migration

I’m trying to upgrade from v.2 to v.3, but there’s something about my list that the system is rejecting.

First I tried the built-in migration link. It got to 59% and croaked. Then I tried exporting my list from the v.2 dashboard, then importing it into the v.3 dashboard. Somewhere along the way it just fails and I get a 500 server error.

I experimented with another newsletter plugin, Jackmail, and it imported the list with not issues. (I decided to not use it for other reasons).

I’m running WP 4.9.8, PHP version 7.0.7.

What are the possible errors in my list that could be breaking it? It’s only a few hundred lines long and I’m content to edit it manually if I know what I’m looking for.