I've disabled all plugins on the individual child sites, as well as all network activated plugins. I've also removed all mu plugins and reverted to the most recent version of twenty-thirteen theme (deactivating my child theme of it). Yet the following error continues to present itself (as I said only on child sites created by new blog templates + pro sites):
Warning: Illegal string offset 'file' in /home/XXXXX/public_html/wp-includes/functions.php on line 3373
Initially there were additional errors occurring as well, which stopped after I disabled the SSL Insecure Content Fixer plugin, so I'm wondering if there is still something active in the database from that causing the problem...
I noticed that pages on which the errors occur are ALWAYS pages that include a picture, and that if I delete the picture the error stops being displayed. Images within posts don't trigger the error, just within pages. Again, this is only on child sites created by pro sites + new blog templates -- the child site that is used as the template is fine, no errors.
I have child sites created by pro sites + new blog templates going back several years -- and ALL of them on the network are displaying these errors -- just since the upgrade to wordpress 4.4
Strange eh? Any ideas?