Plugin malfunction at only one site of multisite network - redirect login plugin

I've using Peter's Redirect Login plugin on multisite. For some reason, at only one subdomain, it won't record new settings. It works at the main site and other subdomains - at least others that I have checked. I get the attached error message. I've performed a limited / partial plugin conflict check, to no avail (I check only plugins active at that subdomain that were not activated on the subdomains where the redirects plugin functioned properly). I've enabled support access in my WPMUDev dashboard. Wanna check it out?