If the problem does occur with twenty fifteen, then it is one of the wordpress installation files that has this issue and not any of the themes. Do you remember what files you modified, if any. I am sure at least the wp-config.php would have been modified. Just would like to know any other file that you or others may have modified.
I am sorry – I do not find any syntax problems with the file itself. Is there a way you can reinstall the wordpress itself (just replace the core files)?
Or better just setup wordpress in a fresh directory (a subdirectory of current server and make it private) temporarily and see if the error gets repeated. If it does not, there is a fair chance that it will get corrected with the refresh of the old one. It’s really hard to pinpoint exactly which file has issues or whether things are wrong in the setup.