User Sync doesn't work with SSL

I'm trying to use the User Sync plugin to sync from a master WP install to two other installations (/store is master, /blog and /plus are the others). The sites are local development sites with a self-signed SSL certificate, so they're not accessible from the outside world. If I turn off SSL, the sync works just fine, so there's definitely something going on there.

I put the plugin into debug mode to get some logging and the log is extremely vague with no actual error message other than "something went wrong". This is the entire log file from the /blog installation when I try to connect it:

[2017-03-03 18:48:51][S] 01 - new subsite conection - central_url=;; replace_user=1;; overwrite_user=0;;***
[2017-03-03 18:48:51][S] 02 - sending request - url=;;***
[2017-03-03 18:48:51][S] 04 - sending request: something went wrong***