I am running the latest Wordpress v4.9.4 and the latest Polylang v2.3.3.
Your plugin worked perfectly until today for some reason.
Not sure what triggered it, could have been an update I did to the latest version of Polylang, but I only saw the error today - this error:
Polylang Slug requires Polylang v1.7 and WordPress 4.0
I also have Yoast SEO installed, so there might be a conflict. I disabled Yoast SEO and the problem disappeared. I reactivated it and it did not come back. It's something weird.
I am running the latest Wordpress v4.9.4 and the latest Polylang v2.3.3. Your plugin worked perfectly until today for some reason. Not sure what triggered it, could have been an update I did to the latest version of Polylang, but I only saw the error today - this error:
Polylang Slug requires Polylang v1.7 and WordPress 4.0
I also have Yoast SEO installed, so there might be a conflict. I disabled Yoast SEO and the problem disappeared. I reactivated it and it did not come back. It's something weird.