Maybe now it works...
A lighting crossed my mind while I was pooping...
And I remembered having activated it in its time: Standard HTTP Caching
https://www.digitalocean.com/communi...n-ubuntu-14-04
In which I remembered that there were some lines, like these:
CacheLockPath /tmp/mod_cache-lock
CacheEnable disk
I had added them to the virtual host file in the first site, just to try, and when I added the new domains (copying the config file) I copied them to all new sites...
I removed them from the .conf files and for the moment everything seems to be back to work.
I restarted the server, but I probably have not freed all the space used by these caches, I have to understand how to do it... and disable them permanently...
Check in the next few days to see if everything works...