Might be a way to find the solution - I am not able to, though.
I've managed to gain access to the backend by deleting plugin files and editing the database (active plugins in the options table). After that, I was able to filter all malfunctioning plugins (including W3) and deinstalled them. I also was able to fix the 404 error in nearly all links.
So at the moment, the site is running - without any podcasts, though. Reinstalling Podlove or the PL Subscribe Button reproduced the error (backend inaccessable, error message in Line 61).
Anyone have an idea how I can fix this?
EDIT: Both lines with errors use "register_setting"...