Is this an upgrade or a new installation?
I suspect you are upgrading from a previous WeBWorK version due to the nature of the error. Check your configuration files for differences from the dist files. It looks like you have some configuration value that has changed type and you have an old configuration value that is not compatible. I would check localOverrides.conf carefully, as I would guess that would be the most likely place for this. But it could be another file also. If you use LTI or another authentication method, then those configuration files are likely as well.