MyBB Community Forums

Is this error after update a serious problem?
The forum seems to work well, also the Google SEO plugin.

you can edit the file & replace THIS_SCRIPT with 'THIS_SCRIPT' (i.e. wrap with single quote mark)

note: take care about Byte Order Mark while editing php files

no, just ignore it, or add a THIS_SCRIPT define to install/upgrade.php

This can be ignored as @frostschutz pointed above. Quotes shouldn't be added as pointed by @.m. .