Hi there Odd Duck,
Any update on this? I'm having the same problem. Getting 500 internal server error when activate plugin.
I'm using the X WP theme & found this (below) on forum. Have implemented suggested settings, but still nothing.
'Make sure it’s CSS Tidy, then following these config:
Disable inline JS & CSS
Enable ‘combine only’
Uncheck pretty much everything else on the W3 Cache minify page.'
Seems the problem could be linked to the 'minify' settings....
Let me know if you have any luck getting this resolved.