That’s right, I opened this thread stating that I had just upgraded the plugin to the latest version, but I had also upgraded WordPress.
Prior to WP 4.4 there was no ‘wp_http_referer’ variable in the edit url of categories and taxonomies, now they added it, and this, for some reason, broke the edit url of The Event Calendar’s custom taxonomy.
Thank Eric for linking the forum thread on theeventscalendar.com, I don’t know why I didn’t look there before posting, sorry.
Anyway, the problem is that the ‘&’ character before the ‘taxonomy’ variable is erroneously encoded, thus causing the variable name to become part of the ‘wp_http_referer’ value.
Why this happens is really puzzling me…