The admin password is stored (as hash) in "webadmin.cred". Just move/delete it, clean the relevant Cookie (or use Private Browser Mode) and set a new one upon loading the admin page.
Ok, after further testing it does appear to work. I found a comic with a couple of double pages and the buttons work as intended. Still testing PDFs.
I am seeing the same on current Chrome & Firefox. CBR & PDF. (Server: Synology DSM 6)
Customer support service by UserEcho