Internal Server Error when changing Advanced

Hello,

I’m using the plugin (it’s very useful), but I’m having 500 Internal Server issues when I try to save changes under the Settings -> Advanced tab.
Saving changes under all other tabs seems to work apart from this one.
I’ve tried looking at the debug.log log file like many suggest but I don’t find anything within it.

Some plugins I have installed are:
ContactForm7
A gallery plugin
PDF Embedder
BoldGrid

I’ve using a default theme, with only a minor adjustment to the footer.

Any suggestions on what could be going wrong?