Fatal Error Call to undefined function SearchApiViewsQuery():placeholder
Have you tried using the Realname module on your site? I'd suggest doing it on a copy of the site to test first as it affects quite a lot of things, but if you're doing searches for names it may help, sounds like search api is getting confused trying to join the two fields, so perhaps a different approach may help.

https://www.drupal.org/project/realname

HTH

Steve