these were all simple text fields.
I refactored the code now and everything works differently now, so I can’t tell you how it went wrong in full detail.
All I know is that – as described above – if the call to
wp_cache_get() fails (for whatever reason), the whole field doesn’t work.
Maybe a simple sanity check would fix this issue here – as a precaution to “not fully trust the WP cache”.
(I also posted this as support ticket, as I actually missed the notice on top before sending)