I just realised that is actually the problem I have when Page Cache Debug Mode is ON. (same error message from wp_debug).
However, when Page Cache debug is off, page cache doesn't seem to work at all. The pages are being saved in my files, but they're not being served, I get a status "200 Ok" in the first of the network requests list on firebug.
Good to know issue with debug mode is already being adressed, but was this second issue expected to happen as well or is it just me?
Edit: by the way, also if I load any page from a secondary domain I have (which is not the one set up in the wordpress db) the page is cached and delivered fine!